@cyrano The "problem" (actually, the feature) with those censorship algorithms is that they rely a lot on the "exact contents" of the message ("Scunthorpe Problem"), so X is probably programmed to detect the Signal's domain and block due to the presence of such link (similar to how Facebook was/is blocking links to the largest PixelFed instances, and then they also decided to block links to DistroWatch and official websites from various Linux distros), so it's not programmed (yet) to censor just the "hexadecimal/base64/whatever" portion of the link alone. And there's where Tox could shine: a handle is literally a hexadecimal sequence, without Tox's domains, without URI Schemas, just a bunch of digits and letters from A to F.
I don't know why Tox isn't mentioned as a "instant messaging platform for whistleblowers": it got Onion (Tor) tunneling possibility (as well as tunneling it through I2P outproxies because it actually accepts any kind of SOCKS5 proxy), it's registration-less (even Matrix needs registration) so it's effectively anonymous IMO.
SimpleX seems to be that, too, although I didn't have the opportunity to use it more than I used Tox. But from the little I've used it, it's similar to Signal in the sense that it's a link (and a large link) and not simply a hash/hex sequence.