Think of this for a second:

If you delete a message on Twitter, the only proof someone else could have is just a screenshot that you can of course deny that it was yours

If you delete a message on Nostr (even if all relays do that), it just takes one person to have stored it and they'll have your signature. You can't deny anymore.

So don't assume "Delete" is a meaningful thing here... let's assume you are speaking outloud and anyone can record what you say.. so you can't undo that for real with certainty