UI scaling not found on unity linux

Yes, this feature is Windows-only. At least for now. Macs also doesn't have it.

As an alternative, you could try the following environment variable from GTK toolkit:

 env GDK_SCALE=2 ./UnityHub.AppImage

It will make all Unity UI elements much bigger. I've tried it and it becomes too big on my 15.6'' 1920*1080 monitor, but it might be better on yours.
Unfortunately this GTK scaling env only works with integers, so there's no way to set it to e.g. 1.5.
You could also try to set an additional env to lower the text size in the scaled UI:

 env GDK_SCALE=2 GDK_DPI_SCALE=0.5 ./UnityHub.AppImage

The GDK_DPI_SCALE=0.5 will make text a normal size while the UI elements are scaled to x2. And, e.g., 0.85 will make the text a bit larger than normal.