Been a long time coming but finally here! There is now an Extra Keys for Linux.
Just one edition equivalent to Extra Keys Pro on Windows. There is no ultra-simple basic edition as I didn’t feel it had much appeal on Linux. Also I first made this Linux version primarily for my own personal use and I mostly use the Pro edition. The only significant difference from the Windows edition is that it does not have the option to type into a named application window, it just enters characters into the active window exactly the same as the physical keyboard.
This tool will not work for everyone because it needs XLib functions for Xorg (X11) so probably no good for people using Wayland as the display manager. However, after several months on my daily-driver MX Linux system and testing on quite few others, it is now available for public download.
It appears to work on most popular current distros, provided they are still using Xorg. As I favour Xfce this is not issue for me at the moment. I don’t run any systems with Wayland so it is uncertain if the program might function with a compatibility layer (Xwayland). So far I have found no way to provide the Extra Keys functionality directly under Wayland, but that is a subject for further investigation sometime in the future!
More details on the main website page – extrakeyspro-linux.html