Right.

You might not want events from the past to have valid signatures in the first place. This relates to the whole "deletion" debate and the "boosting/reposting" debate.

I think the logical thing is for events to have an expiry date attached to them, set by the user when signing. Viewing how short-lived conversations are defacto right now since they just "scroll off the edge off the scren" anyways and 3 days later no one remembers what you said anyways that's just making it official. If you want your note to keep living, you'll have to repost/boost it and it gets a new expiry date. Client will have to provide UIs to maybe automate this client side for the author and provide the reader with clues as to what's reposted vs. genuinely new.

If you regret writing something well just don't renew it and it'll lapse and there's your deletion.