To reproduce:

1. Create a vanilla 3.11 site and install the attached plugin
2. Create a course
3. Turn editing mode on
4. You should see a nice notification telling you that you have turned editing on
5. Upgrade the site to 4.0
6. Go to the course and turn editing on
7. You no longer get the message

This happens because the core_change_editmode sets $USER->editing to a boolean. When historically it's been an integer.