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.
chinese buddhist monks were vegan at least as far back as the 6th century - you can definitely go back quite far
I think there's variants of vegan cooking throughout Buddhist monasteries in Asia. China may be one of the oldest traditions of it, though.