Ordered a few bottom brackets; I've never seen this ring before. I can imagine its purpose, but that imagining takes me to consider that this could be problematic (bottom out before bolt is tight). So - what gives and is this new?
You mean the shoulder that is inboard from the square taper?
If the crank manages to snug onto the square taper far enough that it hits the shoulder but remains loose, then the crank arm is worn and needs replacing.
The tolerances will not allow a good crank to press hard against the shoulder, so don't worry.