Why not delete them?

All notes will get deleted, at some point, except for archive relays. And probably even there, If the data set gets big enough.

Usually, the deletion happens because of some cronjob on the server, that just deletes everything older than XYZ date. Especially chats, Kind 01, emojis, and other low-value posts, as well as notes that have received a deletion request.

But this means that the same note can reappear, if someone rebroadcasts it to the relay again, which is means you:

1) have to keep redeleting the same old crap, over and over

2) have to retain the deletion request, in case someone rebroadcasts a note an npub has sent one for, without rebroadcasting the request that goes along with it.

If notes have expiration dates, you can delete them faster and never accept them, again, because you can refuse to write expired notes.