These files are not part of the actual renderer but part of the system code.
This means, for separated modern and legacy GL renderers, there still will only be one set of this, unlike everything else.
This was done to clean up the license and to ensure that any commercial fork of the engine has to obey the far stricter requirements concerning source distribution. The old license was compatible with GPLv2 whereas combining GPLv2 and LGPLv3 force a license upgrade to GPLv3. The license of code that originates from ZDoomGL has not been changed.