Point is that a STOL aircraft is an advanced flight regime for the average pilot, and therefore he should be taught the technique at the time of conversion.
As for flaps, they were originally called “Landing Flaps” and should be used on every landing, as the folks who designed the aircraft intended.
If the aircraft does not have flaps, then the sideslip technique should have been taught, once more at time of conversion. Most flapless aircraft are “Tail Draggers” which should have also brought to the attention of the pilot, that rudders can sometimes be more than mere footrests.
Having said all that, I have seen instructors doing “Jesus Creepers” in heavy metal jets in central Africa, in the belief that they are showing how a short field landing is accomplished!