Many instructors coming out of sausage factories these days would not know how to sideslip. They were never taught it.
Tell me about it.
I was a PPL student with one such an instructor. At that time I regarded him highly, being the lowly student that I was.
Anyway, one day we ended up way too high on approach (my fault). Instructor said we were not going to make it and I told him I agreed, but would he mind if I tried anyway?
So I threw the PA-28 into a full-flaps, full sideslip at almost flap limiting speed and achieved the 45 degree slope I talked about earlier. We made the runway easily, even touching down in the usual spot and vacated halfway down.
Instructor didn't say anything about it afterwards.