Well back in the day on the venerable old Hawker 801 we used to routinely shut 2 down to save fuel once we were light enough to climb up to safety altitude on one engine, did it on most flights. Otherwise at least one was shut most of the time.
I have also shut an engine down to isolate a fuel leak and continued on 3.
One thing to be aware of when writing manuals, procedures and permissions in modern EASA land is that if a crew are required to to normally, not an emergency, carry out a non-normal procedure they must be Maintenance Check Flight trained. Whether this applies in the case mentioned above will be an interesting discussion to watch from a distance.