some relays literally don't have it implemented

it requires some extra pieces to work, you have to keep tombstones of the event id to prevent it coming back, and that necessarily implies an eventual need for garbage collection in general

and yeah... expiry... i'd implement that on my relay if i knew of a client that i want to use that has it, because that's a cool feature, especially for DMs... it can also function like a dead-man switch too, as part of a system to enable stale follow npubs to be removed (because those damn follow lists seem to be like ratchets, only going in one direction, bigger, because if people don't post they don't see and don't think of deleting