Su DongPo wrote:I find it hard to believe that this change in the Vinaya was sudden, across traditions, and uniform over the centuries. Was it, or are you referring only to Tendai?
Believe it or not, Saicho got the idea from China where during the Tang Dynasty there were many people advocating that the Vinaya be abandoned.
Take for example Daoxuan's remarks here:
《四分律刪繁補闕行事鈔》卷2：「今時不知教者。多自毀傷云。此戒律所禁止。是聲聞之法。於我大乘棄同糞土。猶如黃葉木牛木馬誑止小兒。此之戒法亦復如是。誑汝聲聞子也。」(CBETA, T40, no. 1804, p. 49, b27-c1)
In present times many of those who do not know the teachings destroy and injure themselves saying,"The Vinaya prohibitions are a śrāvaka teaching. In our Mahāyāna we toss it away just like dirty soil. Just like yellow leaves, a wooden cow or a wooden horse deceive a little child, these precept teachings are like this. They deceive you little śrāvaka!"
In Daoxuan's writings he mentions these kind of individuals numerous times. There were also temples which raised cats for hunting mice and engaged in slave trade. Slavery in itself was an unquestioned institution at the time and there were slaves that legally belonged to the monastic institution, but the Vinaya prohibits monks from buying and selling slaves. Again, having slaves is fine if they are donated to the monastery, but buying and selling them is inappropriate. There was a lot of corruption at the time and alternative ideas about precepts and Vinaya. Saicho was merely exposed to these ideas and drew his own conclusions that a monk need not take the śrāvaka precepts.
One somewhat notable school of Chinese Buddhism, which is largely unknown outside of scholarly circles, was the Sanjie-jiao 三階教. They formally advocated abandoning the precepts (捨戒) and formulated suitable doctrines and citations to justify this. It was actually a fairly large Buddhist group in the Tang Dynasty, but they eventually angered the state and were erased from history.
The point here is that Saicho got his ideas from China where at the time some monks were being ordained without the Vinaya and some were abandoning it.
Modern Chinese Buddhists are largely unaware of this. They also don't know that the Japanese idea of "Prajna Soup" 般若湯, which refers to liquor, came from Song Dynasty era monasteries who circumvented the rules by giving new names to banned substances. I don't recall them off the top of my head, but they also had clever names for chicken and meats.