Are there fully anonymous alternatives to Session/Telegram?

OnionShare lets you securely and anonymously share a file of any size with someone.

Tinfoil Chat - Onion-routed, endpoint secure messaging system

Cwtch (/k?t?/ - a Welsh word roughly translating to “a hug that creates a safe place”) is a decentralized, privacy-preserving, multi-party messaging protocol that can be used to build metadata resistant applications.