There are quite a few with a placard "Slips with flaps prohibited"
Cessna 172's? I've never seen one in North America with the word "prohibited", it's "avoid". From a regulatory standpoint, there's a big difference in the intent of these words on a limitation placard.
Yes, FAR Part 23.177 is more restrictive than it's older equivalent CAR 3.118 in terms of the requirement for slips with all flap positions. The 172 is certified to CAR 3.118, which is more permissive.
I cannot speak for Jodels and Moranes, I have no experience with those types.