The idea is that it's left up to the windowing toolkit itself (.e.g GTK or Qt, etc.), so the compositor can focus on just compositing, which makes sense IMO as it's how other platforms handle it (Except they have a single OS provided windowing implementation). Problem is, that leads to massive fragmentation of functionality, every app has different toolbars and features based on the toolkit they use, and requires each app to handle it, which sucks and shouldn't be the case.

Like in the Factorio case, it uses SDL for windowing, and SDL actually supports handling titlebars itself. But Factorio just wasn't including the dependency that enabled it at that point, so all it took to fix it was including it and everything started working. But that's still extra work that had to be done just to get minimum functionality, which wasn't needed on e.g. KDE.

I mentioned in my other response, it's the inflexibility that's the actual problem. Lots of apps do want CSD, or at least control over how their windows are presented, but Gnome going "you're on your own" is the worst outcome.

Their go-to solution is libdecor, which is just a library that implements a titlebar, still putting the burden on apps (Or rather, whatever windowing library they use) to be responsible for it.

Worst thing is, I kinda get their argument against supporting it, they're just really inflexible about it which just makes the whole issue too heated.