...it looks like it's just an Electron app, so all you should have to do is recompile the app with a slightly older build of Electron. (It seems very unlikely they actually need anything in the latest versions of Chromium; you don't need to go back very far for Mojave support.) This should be extremely doable. Maybe a little annoying if it's you first time with something like this, but manageable.
An unofficial Signal wiki still has a “
TODO” on the steps for setting up the macOS correct build environment for simple-minded people like me who can’t code their way out of a shoe box, or else I’d have tried…
…no, wait: I
did try in December, right when they informed users how the desktop client would cease to work/sync on Macs running Mojave or earlier. The basic, I’m-a-simple-brain method of configure, make, make install, didn’t even make it out of the configure step. At that moment, it was also not something I had unbroken hours to pore over and figure out, so the notion of trying again sort of languished.