The questions you have asked are difficult to answer.
For example although i know that it is not followed nowadays but how i am going to prove that?
Similarly the question "When did they stop?" is unanswerable IMO.
As far as the "Why" part is concerned, we can still answer it.
The book " Essence of Dharma Sindhu" says that many forms of sacrifices are prohibited to be performed in the Kali Yuga. And one such sacrifice is the Go-Medha, where a Cow is killed.
Kali Yuga Nishiddhas:
The following are the avoidable and unnecessary acts of Kali Yuga viz. Samudra Yatra, carrying of
Kamandulu to misdirect the Public , remarriages, begetting children
fro m husband’s brothers; Go Vadha; Mamsa Bhakshana at Shraddhas;
performing vivaha to a physically imm ature girl; Chirakaala
Brahmacharitwa; Naramedha Ashwamedha Gomedha Yagna; Madya Paana,
Abhakshya Bhakshana, Apeya paana, Agamyaagamana orcwandering aimlesly
Rahasya Prayascittha, Devata Puja and Pashubali for evil ends; Kula
Bhrashtata; Extreme Profiteering and narrow mindedness.
In general (IMO), all forms of animal sacrifices are needed to be stopped now, as only some very adept persons are capable of performing them exactly as prescribed in scriptures.
The person who's conducting the slaying and sacrifice must be spiritually advanced too. Otherwise, it just becomes an act of ordinary killing and in that case both the Pashu and the person go downwards.
And, that's why, i think, Sri Sarada, the wife of Sri Ramakrishna, abolished the practice of Pashu-bali from the concerned Temples entirely, as soon as she arrived there.