Yep, but that idempotency caching layer would need to share keys across all instances, possibly even geographically. This is a solved problem at the applicatio

Yep, but that idempotency caching layer would need to share keys across all instances, possibly even geographically. This is a solved problem at the application layer of course, but adding complexity at the higher (LB or proxy) layer ... well, adds complexity there and the benefit is not clear to me.