It's an idea that comes and goes. Hopefully with the new simplified NIP-65 with an inbox marker for private relays like that people start to publish relays that can actually support it.
Discussion
we actually built a relay couple of months back with NIP-42 auth and only allows the p tag to download DMs. it is still online wss://securedm.current.fyi it needs the pubkey to be registered with current domain. let me know if you need a key. i can DM it to you.
this will be the error notice: Subscription rejected: Subscription requires #p or authors filter of authed pubkey
Nice! Do you have the code available somewhere?
yes. it is forked version of nostream. check here. Not sure if this branch has the changes. will check and update next week https://github.com/lightning-digital-entertainment/nostream/tree/NIP29