> Branching strategies like Git Flow popularized this approach, and it's extremely common in open source software, where typically community contributors do not have mainline access, and so much create branches and make pull requests

No? I thought Git flow had feature branches, but those were not supposed to live long, just like in every case. Long-lived feature branches should get long-lived by necessity (someone working a long time on a feature that can't or shouldn't be cut down), not by choice. In open source, feature branches are long-lived because either they don't get reviewed or the contributor lost interest/does not have the time to finish it. They didn't choose to make those branches long-lived.

Maybe I just misunderstand what "long-lived" means in this context.