There was an earlier item requesting the option to disable object unlocking for guest editors. As far as I can tell, it was implemented as simply disabling the object unlocking - not providing an option to turn that on and off.
Now that the new option “Only I can unlock” is rolling out, I would really like that option so that I can re-enable guest editors to unlock items other than those with the new “Only I can unlock” option. This gives the most flexibility for those of us running classes with lots of guest editors.