This article shows how to use a keyboard shortcut to set a window to be always on top, on GNOME, MATE, Xfce, Cinnamon and KDE Plasma desktops, as well as a generic shortcut that works with any EWMH/NetWM compatible X Window Manager (so this generic solution doesn’t work on Wayland).

Source: LXer – How To Use A Keyboard Shortcut To Toggle Always On Top On GNOME, KDE, MATE, Xfce and Cinnamon Desktops