Doing it as preference is probably the only way to effectively do this given the decentralized nature of the platform. Disappaearing content requires centralized control,and Imus why there is no disappearing email.

Relays ignoring the preference would be akin to someone screenshotting tweets or snaps. Once you share something it's never 100% under your control anyway.

The one alarming thing with Nostr though is that while screenshots can be fabricated, and thus aren't always trustworthy, your Nostr notes are cryptographically signed, so you lose that deniability.just worth being aware of.