Altered permissions.py code

wagtail_hooks.py

from wagtail.contrib.modeladmin.options import (
    ModelAdmin, modeladmin_register)
from .models import SiteSettings , GlobalSettings
from django.contrib.auth.models import Permission
from django.utils.translation import gettext_lazy as _
from django.urls import reverse, path, include
from wagtail.admin.menu import MenuItem
from wagtail.core import hooks

from .permissions import permission_policy

class GlobalSettingsAdmin(ModelAdmin):
    model = GlobalSettings
    add_to_settings_menu = True # or True to add your model to the Settings sub-menu

class SiteSettingsAdmin(ModelAdmin):
    model = SiteSettings
    menu_label = 'Other Content'  # ditch this to use verbose_name_plural from model
    menu_icon = 'cog'  # change as required
    menu_order = 200  # will put in 3rd place (000 being 1st, 100 2nd)
    add_to_settings_menu = False  # or True to add your model to the Settings sub-menu

class RedirectsMenuItem(MenuItem):
    pass
#    def is_shown(self, request):
#        return permission_policy.user_has_any_permission(request.user, ['add', 'change', 'delete'])


#@hooks.register('register_settings_menu_item')
def register_redirects_menu_item():
    return RedirectsMenuItem(
        _('Global Settings'), reverse('sitesettings_globalsettings_modeladmin_edit', args=(1,)), classnames='icon icon-cog', order=800
    )


#@hooks.register('register_permissions')
#def register_permissions():
#    return Permission.objects.filter(
#        content_type__app_label='redirects', codename__in=['add_redirect', 'change_redirect', 'delete_redirect']
#    )


# Now you just need to register your customised ModelAdmin class with Wagtail
modeladmin_register(SiteSettingsAdmin)
modeladmin_register(GlobalSettingsAdmin)

Try adding this:

class GlobalSettingsMenuItem(ModelAdminMenuItem):
def is_shown(self, request):
return False

class GlobalSettingsAdmin(ModelAdmin):
model = GlobalSettings
add_to_settings_menu = True # or True to add your model to the Settings sub-menu

def get_menu_item(self, order=None):
    return GlobalSettingsMenuItem(self, order)