My Sundowner has a 30 second slip restriction in the POH to avoid uporting the fuel tanks.
That said, a well-executed approach should not require a forward slip. However when executing a forced approach, it is necessary to come in a little high on final...better to hit the far fence at low speed than the near fence at high speed. Therefore coming in high, then dropping down in a forward slip once you are assured of making the field, is an acceptable technique.
Of course it is a manoeuver that should best be practised in controlled conditions. And any POH restrictions should be adhered to.
Forward slips were certainly part of my PPL syllabus and flight test many years ago.
Beech