People reproducibly build Signal all the time. There's a bug right now that makes the play store version differ from the one you get by downloading off their website/build from source, but you can examine the differences to see they're minor.
Pick a decently up-to-date fork of Signal on GitHub and look at its Actions. You can also just do it yourself if you'd like, the process is effectively just doing a build in a docker container and comparing the result.
