A proper private relay will require auth from your nsec to read and write, and you could also run it only locally, use a firewall to limit your IP
If you host anything that requires a username and password, it can be replaced with nostr AUTH
Discussion
Okay... I can prevent others from read/write on my relay by only allowing certain npubs to do so, but how do I obfuscate my relay if I'm someplace like Brazil, who wants to shut me down (I'm not in Brazil, but using that as an example). I would like to be able to hide my relays from being visible at all.