It appears that Bear does not accept contributions[1] and the very few contributors it had in the past only contributed a trivial amount of code[2].

But you're right, relicensing requires the approval of all copyright holders, and in general there can be many. Of course many projects require the prospecting contributor sign a CLA where they relinquish their rights to the project in order to be able to contribute. Personally while I have signed some CLAs, such as the Go one where I retained my rights, I'd never sign one which required me to give away my copyright rights, precisely so they wouldn't be able to do a rugpull on me.

I believe that copyright law is the biggest weapon one has against open source rugpulls and one should not give it away.

[1] https://github.com/HermanMartinus/bearblog/blob/master/CONTR...

[2] https://github.com/HermanMartinus/bearblog/graphs/contributo...