keymap.fullscreen alternates between being described as only f (matplotlibrc.template, comment in backend_bases.py), only ctrl+f (navigation.toolbar.rst), and both values (which is the actual current default) (classic.mplstyle, rcsetup.py).

Simplest would be to make it both values everywhere although I wonder whether we could get away with sticking to only one of the two keymaps.