[immibis]

For E2EE there is the very old unofficial and only-partially-secure extension of using Blowfish with a static key.

[ErroneousBosh]

I guess it's not end-to-end, it's decrypted on the server.

Presumably if you want to send an encrypted message from one literal endpoint to another, you'd use some other technology. I'm prepared to bet there are enough people doing just that, too.

[immibis]

The extension I just mentioned is E2EE.