The relay can still delete those timestamps and the events themselves.
Discussion
The relay can delete the events, but not the timestamps if they are on the bitcoin blockchain.
Who cares about the timestamps hashes in the blockchain if you can't find the event that explains what they were referring to?
Also, the timestamps are for the group of events. So, you need to have the source events to even check if the timestamp includes that specific event or not.
What I am thinking of here is a personal relay, which keeps all your own notes (and others that you wish to preserve), and publishes the timestamps for proof.