Ancient linguists recorded changes in Sanskrit, while simultaneously holding on to "eternal Sanskrit" theology. Similarly, practices like Ashwamedha Yagna seem to have been conducted in the past but have died. Do any scriptures or secular works talk about Hindu practices that have been discontinued/changed/simplified?
Yes , Hinduism scriptures do record changes in its practice . This answer is related with change of practice in Yajnas or Sacrifices.
So let us see these two changes in practices of Hinduism from scriptures. These two changes in practice of Hinduism is mentioned in Shreemad Bhagvat Purana .
offering the remaining part of Yajna Or Sacrifice to Lord Mahadeva or Lord Shiva or to Lord Rudra as his share . .
This change of practice of offering the remaining part of Yajna to Lord Shiva /Mahadeva /Rudra is from the famous story of Daksha yajna as told in Bhagvat Purana.
After Virabhadra's destruction of Dakshas sacrifice in which he did not invited Mahadeva , Brahmadeva and other gods went to mount kailash (the abode of lord Shiva ) in order to pacify him . They assured him that from now on , Lord Rudra/ Mahadeva /Shiva will be the eligible receiver of his share in the yajnas, and the remaining part will be his . Thease are the verses teling us about this change/alter in the practice of yajna.
एष ते रुद्र भागोऽस्तु यदुच्छिष्टोऽध्वरस्य वै ।
यज्ञस्ते रुद्र भागेन कल्पतामद्य यज्ञहन् ॥ SB 4.6.53॥ - See Page 454
eṣa te rudra bhāgo ’stu yad-ucchiṣṭo ’dhvarasya vai
yajñas te rudra bhāgena kalpatām adya yajña-han
53 O Rudra ! Whatever will Remain after the sacrifice shall be your share . Oh Destroyer of the sacrifice let this sacrifice be consummated today with ,this share assigned to you.
This change in practice is also reflected at many places in same Purana.
There is story of Nabhag and Ambarisha in Shreemad Bhagvatam , Where it's again mentioned that once all Rushi's & Deva's in Daksha Yajna decided that , the remaining part of share of Yajna w rightfully will be belonging to Rudra
यज्ञवास्तुगतं सर्वमुच्छिष्टमृषयः क्वचित् ।
चक्रुर्हि भागं रुद्राय स देवः सर्वमर्हति ॥ SB 9.4.8 ॥
The father of Nabhaga said: Whatever the great sages sacrificed in the arena of the Daksa-yajna, they offered to Lord Siva as his share. Therefore, everything in the sacrificial arena certainly belongs to Lord Siva.
- # And Rishi Chavana enabling the Ashivini Kumara the Physicians of gods s to receive part of the sacrificial share . i.e. Yajna Bhaga.
There was practice of not allowing share of offerings in the Yajnas to Doctors OR Physicians. And gods excluded Ashvini kumaras from receiving their share before.
अन्वजानंस्ततः सर्वे ग्रहं सोमस्य चाश्विनोः ।
भिषजाविति यत् पूर्वं सोमाहुत्या बहिष्कृतौ ॥ SB 9.3.26॥
anvajānaṁs tataḥ sarve grahaṁ somasya cāśvinoḥ
bhiṣajāv iti yat pūrvaṁ somāhutyā bahiṣ-kṛtau
Although the Aśvinī-kumāras were only physicians and were therefore excluded from drinking soma-rasa in sacrifices, the demigods agreed to allow them henceforward to drink it.
But Rishi Chavana ,after Ashvini Kumaras restored his youth included Ashvinikumara in the Yajnas and allowed them to receive their share.
सोमेन याजयन् वीरं ग्रहं सोमस्य चाग्रहीत् ।
असोमपोरप्यश्विनोश्च्यवनः स्वेन तेजसा ॥SB 9.3.24 ॥
somena yājayan vīraṁ grahaṁ somasya cāgrahīt
asoma-por apy aśvinoś cyavanaḥ svena tejasā
Cyavana Muni, by his own prowess, enabled King Śaryāti to perform the soma-yajña. The muni offered a full pot of soma-rasa to the Aśvinī-kumāras, although they were unfit to drink it.
So these two practices which were previousely were not there were introduced later on. i.e. Fixing of Lord Rudras share in Yajna and allowing Ashvini kumaras to receive their share in Yajna.
Read S.Dasgupta's A History of Indian Philosophy, available here - http://consciouslivingfoundation.org/ebooks/13/CLF-HistoryOfIndianPhilosophy.pdf. Chapter II says:
THE VEDAS, BRÂHMANAS AND THEIR PHILOSOPHY
The Vedas and their antiquity.
The sacred books of India, the Vedas, are generally believed to be the earliest literary record of the Indo-European race. It is indeed difficult to say when the earliest portions of these compositions came into existence. Many shrewd guesses have been offered, but none of them can be proved to be incontestably true. Max Müller supposed the date to be 1200 B.C., Haug 2400 B.C. and Bâl Ga@ngâdhar Tilak 4000 B.C. The ancient Hindus seldom kept any historical record of their literary, religious or political achievements. The Vedas were handed down from mouth to mouth from a period of unknown antiquity; and the Hindus generally believed that they were never composed by men. It was therefore generally supposed that either they were taught by God to the sages, or that they were of themselves revealed to the sages who were the "seers" (mantradra@s@tâ) of the hymns. Thus we find that when some time had elapsed after the composition of the Vedas, people had come to look upon them not only as very old, but so old that they had, theoretically at least, no beginning in time, though they were believed to have been revealed at some unknown remote period at the beginning of each creation.
The place of the Vedas in the Hindu mind.
When the Vedas were composed, there was probably no system of writing prevalent in India. But such was the scrupulous zeal of the Brahmins, who got the whole Vedic literature by heart by hearing it from their preceptors, that it has been transmitted most faithfully to us through the course of the last 3000 years or more with little or no interpolations at all. The religious history of India had suffered considerable changes in the latter periods, since the time of the Vedic civilization, but such was the reverence paid to the Vedas that they had ever remained as the highest religious authority for all sections of the Hindus at all times. Even at this day all the obligatory duties of the Hindus at birth, marriage, death, etc., are performed according to the old Vedic ritual. The prayers that a Brahmin now says three times a day are the same selections of Vedic verses as were used as prayer verses two or three thousand years ago. A little insight into the life of an ordinary Hindu of the present day will show that the system of image-worship is one that has been grafted upon his life, the regular obligatory duties of which are ordered according to the old Vedic rites. Thus an orthodox Brahmin can dispense with image-worship if he likes, but not so with his daily Vedic prayers or other obligatory ceremonies. Even at this day there are persons who bestow immense sums of money for the performance and teaching of Vedic sacrifices and rituals. Most of the Sanskrit literatures that flourished after the Vedas base upon them their own validity, and appeal to them as authority. Systems of Hindu philosophy not only own their allegiance to the Vedas, but the adherents of each one of them would often quarrel with others and maintain its superiority by trying to prove that it and it alone was the faithful follower of the Vedas and represented correctly their views. The laws which regulate the social, legal, domestic and religious customs and rites of the Hindus even to the present day [early 20th century] are said to be but mere systematized memories of old Vedic teachings, and are held to be obligatory on their authority. Even under British administration, in the inheritance of property, adoption, and in such other legal transactions, Hindu Law is followed, and this claims to draw its authority from the Vedas. To enter into details is unnecessary. But suffice it to say that the Vedas, far from being regarded as a dead literature of the past, are still looked upon as the origin and source of almost all literatures except purely secular poetry and drama. Thus in short we may say that in spite of the many changes that time has wrought, the orthodox Hindu life may still be regarded in the main as an adumbration of the Vedic life, which had never ceased to shed its light all through the past.
Classification of the Vedic literature.
A beginner who is introduced for the first time to the study of later Sanskrit literature is likely to appear somewhat confused when he meets with authoritative texts of diverse purport and subjects having the same generic name "Veda" or "S'ruti" (from s'ru to hear); for Veda in its wider sense is not the name of any particular book, but of the literature of a particular epoch extending over a long period, say two thousand years or so. As this literature represents the total achievements of the Indian people in different directions for such a long period, it must of necessity be of a diversified character. If we roughly classify this huge literature from the points of view of age, language, and subject matter, we can point out four different types, namely the Sa@mhitâ or collection of verses (sam togeher, hita put), Brâhma@nas, Âra@nyakas ("forest treatises") and the Upani@sads. All these literatures, both prose and verse, were looked upon as so holy that in early times it was thought almost a sacrilege to write them; they were therefore learnt by heart by the Brahmins from the mouth of their preceptors and were hence called s'ruti (literally anything heard)[Footnote ref 1].
The Brâhma@nas. [Footnote ref 2]
After the Sa@mhitâs there grew up the theological treatises called the Brâhma@nas, which were of a distinctly different literary type. They are written in prose, and explain the sacred significance of the different rituals to those who are not already familiar with them. "They reflect," says Professor Macdonell, "the spirit of an age in which all intellectual activity is concentrated on the sacrifice, describing its ceremonies, discussing its value, speculating on its origin and significance." These works are full of dogmatic assertions, fanciful symbolism and speculations of an unbounded imagination in the field of sacrificial details. The sacrificial ceremonials were probably never so elaborate at the time when the early hymns were composed. But when the collections of hymns were being handed down from generation to generation the ceremonials became more and more complicated. Thus there came about the necessity of the distribution of the different sacrificial functions among several distinct classes of priests. We may assume that this was a period when the caste system was becoming established, and when the only thing which could engage wise and religious minds was sacrifice and its elaborate rituals. Free speculative thinking was thus subordinated to the service of the sacrifice, and the result was the production of the most fanciful sacramental and symbolic system, unparalleled anywhere but among the Gnostics. It is now generally believed that the close of the Brâhma@na period was not later than 500 B.C.
As a further development of the Brâhma@nas however we get the Âra@nyakas or forest treatises. These works were probably composed for old men who had retired into the forest and were thus unable to perform elaborate sacrifices requiring a multitude of accessories and articles which could not be procured in forests. In these, meditations on certain symbols were supposed to be of great merit, and they gradually began to supplant the sacrifices as being of a superior order. It is here that we find that amongst a certain section of intelligent people the ritualistic ideas began to give way, and philosophic speculations about the nature of truth became gradually substituted in their place. To take an illustration from the beginning of the B@rhadâra@nyaka we find that instead of the actual performance of the horse sacrifice (as'vamedha) there are directions for meditating upon the dawn (U@sas) as the head of the horse, the sun as the eye of the horse, the air as its life, and so on. This is indeed a distinct advancement of the claims of speculation or meditation over the actual performance of the complicated ceremonials of sacrifice. The growth of the subjective speculation, as being capable of bringing the highest good, gradually resulted in the supersession of Vedic ritualism and the establishment of the claims of philosophic meditation and self-knowledge as the highest goal of life. Thus we find that the Âra@nyaka age was a period during which free thinking tried gradually to shake off the shackles of ritualism which had fettered it for a long time. It was thus that the Âra@nyakas could pave the way for the Upani@sads, revive the germs of philosophic speculation in the Vedas, and develop them in a manner which made the Upani@sads the source of all philosophy that arose in the world of Hindu thought.
I would recommend reading all of The first few chapters for an understanding of the development and changes that occurred. As Prof. Gupta says, although some of the more elaborate ceremonials are not carried out with the same frequency as before, the daily rituals are the same. Historical accounts of changes is something that has occurred as a study only in the last few centuries in both the East and the West and was something that was of no real interest of the peoples in either hemisphere.