There's also Migadu which I currently use. It doesn't have all the bells and whistles that providers like Proton and Tuta have with their E2EE but they don't use any shady marketing or track you. Requires a custom domain though.
I recommend switching to a custom domain in the future so you can avoid situations like this from happening again, this way you won't be locked down onto a single provider.
They have a large following on Mastodon so this doesn't really make much sense. I'm inclined to believe that they left due to the hate they were getting because of the recent controversy.
When you use a client, you are relying on the client's crypto implementation to be correct. This is only one part of it and there's a lot more to it when it comes to hardening the program. Signal focuses on their desktop and mobile clients and they hire actual security professionals and cryptographers (unlike the charlatans in this thread) to implement it correctly.
Having third party clients would not definitively mean the client is bad, but it most likely would break the security model. Just take a look at Matrix's clients.
I'm not a fan of GrapheneOS, but the point they bring up here is valid. There is already proprietary firmware on your computer. There's no reason why you shouldn't be updating it to protect yourself from serious exploits. The FSF takes an ideological stance rather than a practical one, unfortunately.
Go to the police immediately!
Well, it's not privacy-focused.. but I do like Revolt for this purpose. It's performant, looks very similar to Discord, and I think they're adding E2EE eventually.
Why do people like Matrix? It's really slow. Even most of the non-Electron clients consume a ton of resources (even more than Electron apps usually do).
Especially Gomuks, by far the worst offender. It consumes nearly a gigabyte of memory and it's a TUI.
I can understand why this may be a issue to some people. I think if they asked Windows users this, there wouldn't be as much of a strong reaction to this. Maybe it comes off as exploiting the good will of the Linux community, but I can't read minds.
I'm personally ok with this. If someone willingly volunteers and enjoys doing this, then what's the problem? But again, I'm not sure if that's the core issue at hand here.
Jokes aside, what practical purpose does this actually serve..?
it's really sad that we've come to asking questions like this.