It’s possible to do it through a Plugin.
Quickly and Dirty, paste and execute in Scripter:
from krita import *
Krita.instance().activeWindow().qwindow().menuBar().setVisible(False)
This will hide the menu bar.
And:
from krita import *
Krita.instance().activeWindow().qwindow().menuBar().setVisible(True)
Will display it again.
It’s possible to create a small plugin on which you can affect a shortcut to display/hide menu bar.
Grum999