Registering custom menu item groups where the "is shown" needs to be evaluated for each request

wagtail_hooks.py

class CalendarViewSetGroup(SnippetViewSetGroup):
    """
    This class defines the Calendar menu, which is only displayed on www and on default sites from other servers.
    """
    items = [EventSeries2ViewSet, EventSponsor2ViewSet, EventTagViewSet, EventSeason2ViewSet, AcademicTermViewSet]
    menu_icon = 'calendar-alt'
    menu_label = 'Calendar'
    menu_name = 'calendar'
    # This puts the Calendar menu just below News.
    menu_order = 110

    def get_menu_item(self, order=None):
        """
        This is where we define the top-level menu object, which lets us hide it based on some runtime criteria.
        """
        class CustomSubmenu(SubmenuMenuItem):

            def is_shown(self, request):
                """
                Only show this menu on www to users with permission to see it, or on the default site to all users
                (since only superusers and super admins can log in to default sites).
                """
                if settings.SITE_TYPE == 'www':
                    return super().is_shown(request)
                else:
                    return Site.find_for_request(request).is_default_site

        return CustomSubmenu(
            label=self.menu_label,
            menu=Menu(items=self.get_submenu_items()),
            name=self.menu_name,
            icon_name=self.menu_icon,
            order=order or self.menu_order,
        )

    def get_submenu_items(self):
        """
        We override this method to customize the list of menu items in this menu, since it needs to contain more than
        just the various ViewSetGroups in its `items` list.
        """
        # Get the ViewSetGroups from `items`.
        menu_items = []
        for registerable in self.registerables:
            menu_items.append(registerable.get_menu_item())

        # Allow other code to register additional menu items through this hook.
        for fn in hooks.get_hooks('register_calendar_admin_menu_item'):
            menu_item = fn()
            if menu_item:
                menu_items.append(menu_item)

        return menu_items


register_snippet(CalendarViewSetGroup)


@hooks.register('register_calendar_admin_menu_item')
def register_all_events_menu_item():
    class CustomMenuItem(MenuItem):
        def is_shown(self, request):
            """
            Show this menu item only on sites which have a MasterCalendarPage2 (which includes only Theme 7.0 sites),
            and only to users with top-level sitemap permission.
            """
            return (
                MasterCalendarPage2.objects.in_site(Site.find_for_request(request)).exists()
                and current_user_has_sitemap_permission(request)
            )

    return CustomMenuItem(
        'All Events',
        reverse('master_calendar_admin:event_planner_listing'),
        icon_name='calendar-alt',
        order=100,
    )