Sadly, it's the modded map_sizes file. I'll remove it for the next mod bundle and maybe create an Issue on gitea. Perhaps someone will work on it (it's pretty low priority as far as the base game is concerned, but is pertinent for mods).
I don't know what the circumstances behind their ban is, or where they got banned from, but what I do know is that this game's dedicated multiplayer community is quite small, and a big part of it is due to the massive disproportionate amounts of skill between active MP players and new people trying to play the game. And, If I'm to gauge this correctly, and assume that they got banned due to slinging racial slurs and generally being a nuisance, I really don't see why allowing someone to go around ruining the experience for people through actively toxic behavior is benefiting the already quite small community in any way, since across pretty much every metric, a highly toxic community is something that draws people AWAY from a lot of games. I also find it incredibly silly to act like them being banned was some sort of overstep. Not to mention, playing 0 AD Multiplayer means you agree to the ToS and ToU of Wildfire Games, and your description of why they were banned clearly shows that they've violated Rule 3 and 4. So what's the point of having a ToS and ToU if those rules aren't going to get enforced?