SMS4Tor

It is very similar to Privnote (requires JavaScript), but not TOR. It is used because it does not require JS and has an .onion address. You have to write a message on their website, create a custom link, and share it with the user you want to send it to. The link can only be used once, so whoever uses it will be the one able to go through the contents of the message. The other option is using PGP, (user friendly) more secure when combined with PGP encryption. This might be difficult to find but not impossible; just Google “self-destruct messages tor”.