Desktop environments that support Super+number out of the box:
Supports the key combo by default. If the app is running, Unity will switch to it. If it isn't, it will be launched.
Ubuntu 18 GNOME
Ubuntu's tweaked version of GNOME supports Super+number, with one annoyance: when switching to an application with more than one window (often the case with Chrome/Firefox), you have to further choose which of the windows to switch to.
GNOME has just implemented Super+N app switching in Jan 2019. I haven't tested it yet.
KDE (no default support, but easily configurable... for a while)
KDE Plasma 5.6.5 doesn't support Super+number by default, despite a ridiculous amount of shortcuts you can configure (Dekstop "20"? Srsly?):
This should be possible in Plasma 5.9.
However, if you go to a Window's "Operations Menu" (click on its icon in the top-left, or press Alt+F3), you'll find
More Actions ->
Window shortcut.... That will let you assign any shortcut you want, including Super+number to activating that window!
You may notice Meta+0 assigned to that action - this is a custom setting from the ridiculous list above, which makes it easier for me to change the 1, 2, 3, ... applications I want to switch to.
Two big downsides:
- After you close the window (or it crashes), the shortcut will be forgotten. You have to create it again.
- If you don't have the app running, KDE won't launch it for you. Unity and Windows will.
An annoying downside of KDE's keyboard handling in general is that it doesn't open the launcher after releasing the Super/Meta/Win key, and that it apparently forgets shortcuts after logging out.
It's a bit mind boggling how broken KDE is anyway, after only a couple hours of playing with it.
DEs with support via extensions/plugins/applets
Note that letting plugin authors build such a feature is fraught with problems. In several years, I haven't found any plugin/extension/macro/whatever that gets this right. This sort of feature needs to be well-integrated into the window manager, and this was confirmed by one of the authors of MATE Desktop, who agreed with me.
Cinnamon has an applet that does switch among open windows with Super+1..9, though with some issues.
DEs that don't support Super+number out of the box