none of the Geany-plugins devs had enough interest in those capabilities or enough time available at that point to examine it
Features that devs have no interest in have low chance of being added. This applies to most projects.
Would rather devs evaluate bug fix PRs than new features few others want.
There are other ways to hide the menu bar:
* Global / dbus menus and libraries. Plasma 5/6 have one included. XFCE has a widget that can be added to the panel. I've seen packages for other desktops, but have not tested them.
* Via a script with the GeanyLua plugin.
so now nobody can try it at all
The plugin is still available in its own repo (link in comment to PR). GitHub also keeps records of all PRs that sufficiently motivated users can use to build.