At least its plugins, which are what needs to be regularly updated as platforms change, are open source. I imagine someone will clean-room reverse-engineer the core app and make an open source one that uses the plugins.
Also, Louis Rossmann said they only reserve the right to go after forks to prevent a situation like all the shady ad-ridden NewPipe ones flooding Google Play Store.
Not non-transferrable, as that would prevent Elon from claiming @america or the transfer of @POTUS.