I remember from my old days at engineering career that we made lots of flux related problems, and I know that when surfaces get into the game the problem is much more complex than when you use only ‘points’, but I cannot imagine why cylinder lights are supported and planes are not. Maybe I’m wrong but I think that complexity on both kind of lights should be more or less similar.
I could spend some time adding support by myself it is easy but maybe someone can give me a reason that illustrates why that’s more complex and it will save my time.
I don’t know that there’s any real reason not to include them, they would be rather beneficial for certain light setups. However, if you really need to do it for any reason, you can often just use the “Static lighting from emissive materials” option and just do it on your texture. Lightmass will take care of the light shape and calculations for you. Of course, that kind of system doesn’t really work for dynamic lighting, so if you need it for that, you’re out of luck unless you get something like VXGI or LPVs set up in your scene.
Self illuminated planes didn’t work nice for me with lightmass neither.