Remove set all tasks complete checkbox from the status close
In preferences not UI Config.

This cannot be changed using UIConfig because it is a 'complex' task with multiple objects

  1. The checkbox that let's them set them all complete or not
  2. A banner that replaces the checkbox when all tasks are already complete

So instead it is a Preference that does everything you want for it.

Like all Preferences, first decide whether you want this change an a Company/Org level, Access group, Repair center or individual, then:

Shows the steps to turn the set all tasks to complete on or off

or you can search for it:

How to find the set all tasks to complete hide or show by searching

Note that the option to BY DEFAULT set all to complete or not is a separate option, as shown in the search above or the image directly below. This other choice will not hide the checkbox, just decide whether it is checked or not by default.

Shows where the default value for set all tasks to complete is

If it is visible and checked, when you hit save on the inset status dialog, all tasks will be set to complete,

Shows the position of the save button in the inset status dialog

and the green banner showing that all tasks are now complete.

Shows the green banner that all tasks are complete