There are still some less-used protocols that offer advantages over traditional protocols.
You have RCS, but it was developed as a cash cow for providers, it is very limited in what it supports and encryption is an optional extra layer. It is generally available.
You also have Matrix.org, a chat protocol where you have end-to-end encryption (with multiple devices, so no proxy over your phone like Signal or WhatsApp). You can host your own server that integrates with the rest, just like email really, or you can use one of the virtual servers. The system isn’t quite right yet, but you have a bunch of different apps to chat with. You can even make encrypted (video) calls, although I can honestly say I’ve never tried it. Mozilla, among others, switched from IRC to Matrix for discussions, a non-controversial decision. It is “new”, in full development and begins with fits and periods.
I’m using it myself, with bridges to use Telegram, WhatsApp and Signal for people who don’t want to switch; It takes some messing around with the servers, but it’s a very good idea to integrate all the chatting apps together. Of course you can specify E2EE with it, because the core protocols are not 1 to 1 compatible.
There is also another E2EE extension on XMPP (many of them actually) that is older than Matrix and has many of the same benefits and more, but encryption support varies by client. The client and server must also support many optional extensions to get close to modern messaging services. If you have it all together, this is an absolutely great system. XMPP is a bit like IRC in that not many people use it, but the old guard will swear by it forever. Setting up your own server is relatively easy, and Mailcow has a built-in XMPP server if you use it to host your mail. I’ve played with it, but I don’t think it works well enough in the end.
You also have Threema, an app that you actually have to pay for so the incentives aren’t wrong. It has the same advantages as Signal, but without the crypto scam that they build in. However, try to convince your friends to donate money for “Another WhatsApp”.
You also have Torchat I think, I tried once but wasn’t impressed with myself. I don’t know about the server platforms and dependence on them.
