Comment 4 for bug 1755831

That picture is indeed unity, but I have used Ubuntu Gnome, Ubuntu 17.10 and Fedora, they all support this feature.
By the way, there're many stuffs in that option, the main thing is key combination, there are two types:
_ 1 character + 1 number
    Ex: 'a' + '1' -> 'á'
        'e' + '6' -> 'ê'
_ 2 character
    Ex: 'a' + 's' -> 'á'
        'e' + 'e' -> 'ê'
Some people are used to the first and some are used to the second way.

Talking about Settings - keyboard, all I can see are shortcuts, after choosing "Reset All", I now cannot use Print to screenshot anymore, and I can't even disable default shorcuts by setting them to <space>.