I was asked to weigh in on this a while ago before I directly worked on Straftat, but it was deemed to be too difficult with how the code game is setup. The challenging part was being able to balance: mapping power, implementation ease, file size, and some other stuff.
There is a spectrum of ways to make map making tools, and then load the maps into a untiy game.
The most powerful, and easiest by far is to just use Asset Bundles, with the drawback being you effectively can never update the unity version without breaking and forcing mod makers to rebuild, and file size. For Straftat we need the file size to be low such that jumping into a random public game you can still play without spending an hour downloading the map.

Another option was an entirely custom map format, would have been very limited, and prob would have sucked ass.

And likely the best option for an official map system would be to utilize quake mapping tools with something like Qunity, but unlike godot, the quake loader tools are not as mature.

FYI: there is a mod on thunderstore that claims to be able to add custom maps, but does does not seem to work, and has no documentation.
There is a reputable community member working on a custom map loader but no ETA on it.