They used to make slide mounts with smaller windows and, I think, even different frame shapes for SOME selection in cropping.
None the less, all my projected slides are shot on 35mm and the vast majority are shot with a zoom. That makes "in camera cropping" much easier.
I'll just happily continue to crop away any time I think it improves the final print and y'all that don't like or approve of that can continue insisting every print you make is full frame then, ok? So everybody is happy?