There is a similar environment variable for Steam on Linux and I would argue that it should be an application setting but for whatever reason it isn't.
Both of these are hacks around how Xorg (doesn't) handle fractional scaling.
I understand that, but your window server is just another application. On Windows I wouldn't consider scale factor being an env var, and indeed it isn't set as such -- rather that value is set in the registry.
Maybe that's the answer -- Windows has a defacto method of setting system-wide env vars that every application inherits within a given personality but Linux/BSD does not.
I can live with that answer :-)
Not the window server.
QT is a library that applications can use to render their GUIs, so there are many instances of QT in all of the apps that use it.