Referring to the link explaining the difference between a side slip and a forward slip, and to some of the posts trying to explain the difference, I can't see any difference, except that in the so called side slip, the amount of slip will usually be limited to that amount needed to off set drift. Whereas the so called forward slip is limited by rudder v aileron/roll spoiler authority and the size of the pilot's gonads.
The aircraft doesn't know or care which way the wind is blowing, both manoeuvres involve crossed controls and a flight path into the lower wing.
For my money, they're one in the same manoeuvre.