Signal is making a part of their server source code proprietary software:

"To keep Signal a free global communication service without spam, we must depart from our totally-open posture and develop one piece of the server in private".

Taking into account it's centralized nature and that they don't allow third party clients, Signal seems to be just as bad as other "privacy" oriented companies.

Leave Signal and join a truly private, decentralized and libre protocol like #XMPP