But only if you were using your disks with single side devices and you also had to make a notch or you couldn't write anything. I was using double side drives and doing this would have erased the data.
That may have been for 5.25" disks... I think you also had to cut a hole in the case for the optical revolution sensor. I've seen it done, but it was very messy, e.g. the bottom of the disk cut open to remove the actual disk so the sensor hole could be cut without damaging the disk.
What the grandparent post is probably thinking of is 3.5" disks, which had an optical sensor and a hole to tell the drive if it was low density (720k) or high density (1.44M). The theory went that they were economising by making the low density disks from rejected high density ones. Either way, you could drill a hole in the things (which risks swarf ending up inside the disk and damaing the heads) or later on, get custom hole-punches to easily and safely cut out the notch in the disk casing.