FWIW, the EU has passed the Digital Markers Act which requires big tech companies to open up their platform. It'll take a while before it takes effect, but unless Apple, Google, Meta and all others are going the very expensive "fine me and we'll see what happens" route, we should soon see movement towards an environment where different apps can talk to each other again.
The tech industry being what it is, there is no real standard for interoperability yet, though there is an IETF taskforce working on setting up a standard companies may follow: https://datatracker.ietf.org/wg/mimi/about/
This, critically, considers encrypted messages in a standard way so that we don't lose E2EE in the process.
https://www.beeper.com/ is promising. It's set of matrix bridge over 15 chat network or so.
Neat.
For instance, I remember hearing that Discord banned users who used alternative clients.
I am astonished by how well the Signal bridge works — everything works! Read indicators, threads, mentions, groups…so far I have it configured as a linked device to my phone, but you can also configure it as your primary device.
I really want to use the WhatsApp bridge, but that seems messier. I don’t want to install WhatsApp on my phone, so it requires running Android in a VM with a VoIP number, and apparnetly risks getting banned from WhatsApp.