Perhaps there isn’t a HL”M for rotzeach regarding a historical date, while there is for animals.
If I were a betting Gavra (or a bit more OO), I would say that Rav Moshe (and the RAMBAM!) agreed that it is unconscionable to allow application of the standard “Halacha L’Moshe Mi’Sinai” Treifos to people, and therefore not apply Misas Bais Din to someone who killed a person that could have lived a full life.
Therefore, although Rav Moshe can accept the “Keveiah” by the Chachomim for animals, it would be impossible for Chazal to have done so for people.
Note that Rashi in Maakos seems to disagree, and implies that if a person had a siman Treifah that the Rotzeach would not be convicted by Beis Din.