No, the whole history of RHEL development remains available on GitLab. The change only affects git.centos.org.
I would assume the people that actually develop those either already are RHEL customers or they would just pay a single subscription so they can download the code and keep it in sync
There are no "official" derivatives of RHEL. (And that was true even after Red Hat acquired/acqui-hired CentOS).
I have edited my question from official to legal (at least from a US-perspective). Another commenter in this thread has stated that it would be against their TOS.