At this point, what messaging app is truly OTR? Signal doesn’t allow third parties to build the source, Telegram is compromised for sure, same with WhatsApp.
No. Signal is GPLv3.
They don't allow third parties to distribute modified binaries that connect to their servers. You can argue if that is a good idea, but don't misrepresent the situation.
The whole spirit of GPL and FOSS is to be able to examine the source, modify it, and distribute it freely. It’s a form of DRM to restrict who can and cannot distribute the Signal binary.
Maybe this is an unpopular opinion, but IMO the fact that Facebook "trust" it means that I have a lot of reserve about using it myself.
(if that's the rule anyway - I couldn't find confirmation for that)
Hence why Moxie doesn't think federation is practical with an evolving communications protocol.
[1] https://matrix.org [2] https://riot.im