Hmmm, forward slips.
Aside from the rather large rates of descent that can be obtained, these maneuvers are not especially helpful when performed close to the runway...say in the last four miles or so, whilst on final, as an example, for excess height reduction.
We had one very senior Captain who used to train (and perform) these forward slips for excess height reduction, on a somewhat regular basis.
And in a 300+ passenger airplane.
Management got wind of this, and contacted the manufacturer, who promptly dispatched a company test pilot out to our location, with actual data obtained from certification testing.
Result?
NO pilot ever again forward slipped the airplane.