In the C172 I fly there is a placard saying:
Avoid slips with flaps extended
Mine too, but I think this refers to the kind of "adventurous" slipping that one would not undertake with full flaps on landing.
ie if the x-wind is bad enough that you have full opposite lock on rudder / ailerons, you wouldn't be using full flap for the landing.
FWIW when I asked about this I was told a) the reason for the placard is that under full flaps a slip can in certain circumstances reverse control input and b) the placard relates to full rather than partial flap. And by full flap I mean the 40-deg "barn doors" that many of the old C172's I fly have. I've never tried it
so can't tell you if it's true or not!