That is my assessment as well. Even those variants which alleviate some of the common complaints don't seem to see any activity. But then, what custom combat map *does* see activity outside of those which are selected to be included in the game?
I usually advise people not to bother with KoTH maps as there isn't much you can do to make an original and engaging layout that significantly changes the gameplay. I think that's why novelty maps are popular. The layout in those often doesn't matter but people seem to like new forms of objectives and obstacles, as well as theming.
I think Attack/Defence maps are a good place to start because they teach people about difficulty ramping, territory control, blocking structures, retreats, flanks, class-suited routes and architecture, etc. and you have a lot more room to experiment and to create something different. But I haven't built a combat map in a long time.