It's only enabled when the document supports multiple page sizes, that
at the time of writing (and since 13 years) is none

So not great to have an always disabled menu :D

The backend part (Document::supportsPageSizes) is still there, so if we
ever reintroduce a backend that supports it part of the code is there,
and probably we may want to rethink the UI so it only shows when useful,
and not being disabled without possibility of enabling it for say PDF