ZDoom

list REKKRSA.wad in the IWAD selection menu, add a separate config for it, don't load the Doom dynamic lights with it, and maybe have the compatibility default to Doom (strict)

Make an .iwad that autoloads REKKR and applies all needed settings. No need to pull a support for every IWAD imaginable into the engine.

ROTWB is supported, why not REKKR?

Because handling of .iwad/.ipk3 was implemented two months after ROTWB was added as IWAD.

And to add to that, these days there are far more custom IWADs than there used to be and adding support for them all would be ridiculous. As a result a system that allows them to be configured by the author now exists. So there is no need for them to be added to GZDoom itself. If it's been properly set up by the modder, it's effectively added already.

AFAIK all that's needed is to rename REKKRSA.wad to REKKRSA.iwad.

And add an [wiki]IWADINFO[/wiki] lump (if there isn't one there already).