Is there any explanation given? It's quite troubling that Red Hat has
chosen to publish the LGPLv2 licensed binaries with source but without
the devel package.
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.
--
Kaleb