Some earlier 172's have an "avoid slips with flap extended" placard, but it's not unapproved........Other 172 models have notes which vary in this regard, but none prohibit slips with flaps extended, or the plane would not be certifiable
Cessna 172H POH - Slips are prohibited in full flap approaches because of a downward pitch encountered under certain combinations of airspeed and sideslip angle.
My guess is this prohibit clause with 40° flap may have been why later models no longer had 40° selectable.