Anyone who willfully uses a slotted screw on a new project in the presence of even somewhat reasonable alternatives should have a lifetime ban on designing any items for anyone.
it’s single valid use: somewhere that you may not have tools but might require an impromptu fix… preferably paired with a much larger screw than technically required so you can use things like coins as a driver
We put slots on the thumbscrews used on some of our connectors for military radios. This was a soldier can open or tighten them down with a knife blade.