Being vegan in 2023 is extremely easy, but to me that seems like a relatively new phenomena.
So if I put you in my vicious time machine and sent you until the dim past, how far back would you be vegan?
I think I could make it work for most of the 1900s. But if I was in 1500 I'd give it up.
https://en.wikipedia.org/wiki/Timeline_of_plant_evolution
probably the late Cretaceous after flowering plants start to dominate and diversify
that's roughly 70+ Mya