my employer uses a lot of paid github features, while my projects use none, so moving away would be simple for me, but difficult for them.
the real lock-in is no different than any other social media platform: network effects. most people who follow you on X rarely bother also having an account on Y. unless you're Taylor Swift, moving away means diminished visibility/reach, and dilution of any reputation accumulated e.g. over a decade of activity, interactions, etc.
