As in the title.
Unreal Editor 2.0 was what UT99 used and it's literally my only experience making maps.
I adored the CSG for how intuitive it was but sadly Unreal Editor 2.0 is not open source.
I know its a very dated technique but I don't mind that at all.
I could not find anything similar to it.
Everything is either too heavy, tied to a specific game or doesn't have CSG.
Any similar editors would be appreciated too.
Even a 3D CSG library in C or C++ would be appreciated, since the only one I found was in Python and had completely inactive GitHub repo and bugs.