@SuperDicq@minidisc.tokyo sort of. The license does allow people to make forks or use the code in other projects.
I was not aware that it uses proprietary libraries. I just installed a foss fork called molly (which gets rid of these libraries, without losing compatibility).

Despite the fact signal relies on proprietary libraries (which seem to be present to interface with google play services, which is only possible if you've got those installed), I still think it is a good substitute for big tech messaging platforms. They are very much against sharing metadata. The fellow in charge used the following situation as an example; someone makes calls to a specific number regularly. One day they make a several hours long phone call to a hospital. Then they never call the number from before again.
The carrier doesn't know what was said during the calls (probably), but the metadata already tells the whole story.
I think this is an excellent example, so I thought it was worth sharing.

Not sure if this is true, but someone told me that signal is holding back cross platform messaging because of privacy concerns in the proposed concepts for the protocol to be used. (iirc the person who told me this said the protocol is being designed by the EU government, but I'm really not sure. I'll have to look into that sometime)