See https://github.com/geany/geany/pull/2972 and https://github.com/geany/geany-plugins/pull/1138.

Oh well, #2972 didn't show in my search because it is closed, and I didn't look for plugins.

Core Geany devs appear to simply not want menubar hiding to be included in Geany.

_Most_ don't, so clearly those are not going to work on it, and since the PR was only open for a month none of those who might accept it had time to look at it. (I guess its Murphys law that the ones who might accept it are those with least time available for Geany :-)

Not sure what the availability of the plugin collection maintainer is, but I'm sure its not great, so if nobody else tried, tested, and supported the plugin it is likely they didn't spend any effort on it.