IIRC, Redhat also made it more difficult to make a RHEL clone in order to neuter Oracle Linux and other possible RHEL clones and the sponsoring of CentOS was to have a cost-free RHEL clone in their direct sphere of influence.
I'm guessing you're talking about Red Hat stopping breaking out its kernel patches individually about 10 years ago?
That was to make it harder for Oracle to offer support for OEL, but made no difference to Centos & the non-commercial clones because they shipped the RH kernel unmodified and didn't need to worry about what any individual patch did.
Source RPMs are also not publicly published anymore. The source is available in the CentOS git repos, will be interesting to see what happens after CentOS 8 ends.
