Hello, future? Yes, this is Richard Stallman calling from the 1980s. I think what you might be looking for is the source code to the entire software stack.
Perhaps a little pedantic, but I don't think this is technically correct, since Stallman and the GPL wouldn't really apply here for server-side code, since it doesn't seem like the client-side application code is being questioned regarding trust, but rather the intermediary code on Signal's own servers. In that case, Affero GPL would be the answer. And that license was first published in 2007, which is something like 18 years after the initial GPL release.
I don't think that actually detracts from gist of your point, but just wanted to point it out in case anyone was interested.
I don't think that actually detracts from gist of your point, but just wanted to point it out in case anyone was interested.