Are sudras allowed to read scriptures other than Vedas ?
Yes they are allowed to read and hear some Scriptures like the Puranas,Itihasas etc.
From Devi Bhagavatam:
18-24. At every Manvantara, in each Dvâpara Yuga, Veda Vyâsa expounds
the Purânas duly to preserve the religion. Veda Vyâsa is no other
person than Visnu Himself; He, in the form of Veda Vyâsa, divides the
(one) Veda into four parts, in every Dvâpara Yuga, for the good of the
world. The Brahmânas of the Kali age are shortlived and their
intellect (Buddhi) is not sharp; they cannot realise the meaning after
studying the Vedas; knowing this in every Dvâpara Yuga Bhagavân
expounds the holy Purâna Samhitas. The more so because women, S’udras,
and the lower Dvijas are not entitled to hear the Vedas; for their
good, the Purânas have been composed
And the reason why they are not allowed to read or hear the Vedas is because they don't wear the Sacred thread.
The Sacred thread ceremony is done only for the men belonging to the first 3 castes but neither for the Sudras nor for the women.
Before reading the Vedas an "initiation" is needed which the Sudras are not entitled to have.
Manu Smriti 10.126. A Sudra cannot commit an offence, causing loss of caste (pataka), and he is not worthy to receive the sacraments; he has
no right to (fulfil) the sacred law (of the Aryans, yet) there is no
prohibition against (his fulfilling certain portions of) the law.
10.127. (Sudras) who are desirous to gain merit, and know (their) duty, commit no sin, but gain praise, if they imitate the practice of
virtuous men without reciting sacred texts.
However,having posted all this,let me clarify that i'm not quite in support of determining caste by birth or by samskara.
Because,in Brahma Purana even Parama Shiva says:
Na yonirnaapi samskaaro na Shrutirnacha santatih, Kaaranaani
Dwijasttvasya vrutthamevatu vidheeyatey
It is not by birth, samskara, knowledge of Vedas and by the Offspring
that a person possesses, but Brahmanatwa is a decisive factor merely
on one’s own behavior, conduct and actions!
But determining caste by Gunas only is also no doubt a complicated and somewhat impractical process.