After playing Endless Riches, when a token is killed, is it banished?

No. When a token is killed under Endless Riches, it goes to the trash as normal — it is not banished.

Endless Riches' replacement effect says "If a card would go to your trash... banish it instead." Tokens are not cards (Rule 182), so that replacement doesn't apply to them. The token will briefly hit the trash, then immediately cease to exist per Rule 183.1, same as any other token death.