They also require running the app on a "real phone" for registration. Most emulators put you in captcha hell
Damn, what a shame... Another vote for Session, I guess. Idk what flaws that one has but it seems to be the leading alternative to Signal.
And you can use your Session ID to post or reply to posts at https://www.LokiList.com (best viewed with javascript disabled) for anonymous casual encounters.
Signal still requires a phone number for registration.