Greetings,
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Troubling? Why is it troubling? Since you mention the GPL, the GPL only requires that the source be provided, and it is.
-devel packages are an artifact of Red Hat's (and Fedora's and CentOS's) RPM packaging. The GPL doesn't mention -devel packages.
And just to set expectations, it appears the libXvMC-devel package will most likely not appear in RHEL's Code Ready Builder or CentOS's AppStream until 8.2.
I was always under the impression that Red Hat does what is in the best interest of its customers... and I fail to see how not providing (some of) the devel packages is in their interest. That's just my opinion and I reserve the right to change my mind at any time. Anyhoo... what has been missing here is the "reason" (some of) the devel packages aren't being released.
It is certainly true that Red Hat has the right to do that, and unfortunate that CentOS will monkey-see-monkey-do in their efforts to be just like RHEL down to the same bugs and problems. You mention in the case of a particular package that the release is planned but not until RHEL 8.2. Given the specificity of that information, again, it begs the question of why?!? I think answering why would remove the confusion. I don't assume there is a dubious reason, but without knowing, all we have are guesses.
TYL,