It seems not possible to make a Formula that does anything useful with a Single Select field.
E.g., If I want to get the Name of a Single Select field’s current selection I need something like:
but I can’t figure out a way to do that.
I assume the same problem applies to Multi-Selects, though I haven’t tried them.
If this did work, it might make it possible to have something like a “Dynamic Single-Select” type, which could populate its options dynamically for each entity, based on a formula. That’s what I was trying to simulate when I ran into this issue.