Under the administrator pages under the menu item “Tasks” there is list of crontab jobs that do various functions and run periodically.
How can I add such a task that will set to true the event_creation_restricted field for all categories but the Test ones?
If this is not feasible, then how can I “disable” users from changing this field?
you’d have to write a plugin for this
Is there an example of such plugins?
have you tried searching for task in our plugin repos? 
there are quite a few plugins that contain periodic tasks. older(!) versions of the foundationsync plugin are probably a good example since that plugin used to be just a single python file.
I just started looking into this, as we trying to upgrade to the latest version and this is one of the things that I need to do in order to maintain the same level of security that we currently have.
I’ll look into the plugins and try to create a new one for this task.
THANK YOU