but I can clearly remember being trained NOT to do sideslips with Flaps.
Isn't it written down in a C150 or C172 manual somewhere?
You have a good memory! Extract from the Cessna 172 Informational Manual at page 4-20 Section 4 Normal Procedures
Quote: "Steep side slips should be avoided with flap settings greater than 20 degrees due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and centre of gravity loadings" Unquote.
The Cessna 152 Information Manual contains no such advice, suggesting the advice is limited to only the Cessna 172.
As with a lot of instruction at flying schools, there are flying instructors who are prone to give sweeping generalised advice, such as for example NEVER side slip with flaps down in any aircraft. Impressionable students then take this as Gospel Truth throughout their flying career