Cessna high wing series 150 /172 is that slips with flaps extended were not approved
C150/152 have no restriction regarding slips.
Some earlier 172's have an "avoid slips with flap extended" placard, but it's not unapproved. The 172N is an example. Though it also has an amplified procedure under "Crosswind Landing" which says:
When landing in a strong crosswind, use the minimum flap setting required for the field length. If flap settings greater than 20 (degrees) are used in sideslips with full rudder deflection, some elevator oscillation may be felt at normal approach speeds. However, this does not affect control of the airplane. Although the crab or combination method of drift correction may be used, the wing low method gives the best control. .....
The wing low method would be the beginning of a slidslip.
Other 172 models have notes which vary in this regard, but none prohibit slips with flaps extended, or the plane would not be certifiable. For the 172S, it's a recommended procedure for a wing fire in Emergency Procedures.
All Cessnas can be slipped with the flaps retracted, which is a good starting point if you want down, flaps can be extended later.