GTK3, assign custom keyboard shortcuts for menu items?

[Stefan Salewski <mail ssalewski de>]

Some years ago I managed to get it working for my Ruby App -- I am not
sure if only for GTK2, think it worked for GTK3 also.

Not that I need or want it myself, but maybe some users of my
application do?

As I remember, it was hovering the mouse over a menu item and then
pressing the desired keys?

Currently that seems to work for none of the GTK 3.14 apps on my Gentoo
box.

Is that feature deprecated now? Can not find much useful with Google for
this topic.