I operated the DHC5-D Buffalo for many years and it was quite normal to make STOL (of short field approach/landing) on a 7.5 degree approach. I cannot think of any reason ANY aircraft could not achieve that except a structural thing ie the tail striking the ground on touch down. The Buff has a very steep rear end but even so the round out is pretty minimal basically just land it FLAT.
However the CAA have had a 'thing' about STOL performance aircraft - I can remember years ago when the Dornier 228 first appeared on the scene the Germans had it certified for STOL approaches and take offs but the CAA whinged and moaned until it was basically killed stone dead.
C (Campaign ) A (Against) A (Aviation) strikes again !
Of course it takes a good set of hands and a steady eye to make a good STOL approach (remember Farnborough and the ex RAF Marshemellows attempt ?) so there are lots of pilots out there probably not up to it!! (Yes very controversial I know but true!)