Client can also just ignore expire date. I mean, you could make YOUR client honor it, but that doens't mean they are gone gone.
The expiration functionality has existed in Nostr for a long time and almost all nostr relays honor it. So yes, you could run a relay that doesn't honor the spec, you can sae a copy of the nostr event, you can screenshot the content. But you'd have to run that off the main nostr network. It's not proof the content will go away, more about expressing intent and most of the nostr software honors that intent.
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