This has, somehow, been twisted and distorted into:
Do not slip a 172 with any flap
Do not slip any 172 at all
Do not slip any high-wing Cessna
Just to be on the safe side, don't slip any aeroplane.
If you do, the flaps will fall off and your willy will turn green
etc...
etc...
I think the confusion is created by Cessna 172 POHs and placards stating things like "AVOID SLIPS WITH FLAPS EXTENDED". In other POHs I have seen it they state "AVOID SLIPS WITH FLAPS SETTINGS GREATER THAN 20 DEGREES", etc. People read "Avoid" as "don't", it's just Cessna playing safe as they were subject of a law suit years ago.