It wouldn’t necessarily have to be the relays that respect it. You could have a NIP that provides an “expiration date” and clients that support that NIP would know when to stop showing that story.
Discussion
Expiration date of events is already defined in NIP-40:
https://github.com/nostr-protocol/nips/blob/master/40.md
No need to define it, just to code clients and relays supporting it.
Nos.social supports disappearing notes. You can even choose between 15 min, 1 hour, 1 day and 7 days in which the note will disappear.
