If I remember correctly the 172P is the only model I have seen a placard in that says anything about slips with flaps extended. The 172S does not have a placard but if memory serves the POH (I may be thinking of the R) recommends against slips with full flaps because of a possible elevator flutter. I don't think it specifically prohibited in any model, just not recommended.
The A through M model had 40 degrees of flaps and the elevator would buffet a little if you did a slip with full flaps in. The N through current are only 30 and don't have the issue. It's not an issue to begin with to be honest.
Actually the N still has 40 degrees of flaps, but starting with the P they only went to 30. I have read that most 172Ns have gone through an aftermarket engine upgrade of some sort- I can't remember what it was- that required reducing the flap range to 0-30. The only 172N I have ever flown, which I did my private training in, had 40 degrees of flaps.
I have slipped 172s with full flaps plenty of times, including the N with 40 degrees, and only encountered the elevator flutter/buffet once (oddly enough in an R with 30 degrees). I would agree it's not all that much of an issue.
Never flown a Cessna during any of my primary training. Honest question, and i'm lazy, but what is the major difference in all of the 172 a,b,c,d,w,t,f?
I myself have flown the L, M, N, P, R, and S models, so it looks like I win this thread so far for the number of 172 models flown

. All of them fly about the same and have the same, but the V-speeds differ by a few knots between each model. I didn't notice much difference between the L,M, and N except the N is the first model to have "pre-selected flaps" instead of the up/down switch. The P has a higher MTOW and only 30 degrees of flaps. The R is the first fuel-injected model and has bigger tanks with 53 gallons usable fuel instead of 39 or 40 like on the early models. It also has 13 fuel sumps. The S is pretty much an R with a 180 horsepower engine instead of 160.
Some of the early 172 models have 6-cylinder continental engines instead of 4-cylinder lycomings, and the earliest ones have manual flaps, but I have never flown one of those. Yes, I know way too much trivia about the 172 but it looks like I'm not the only one based on this thread.