I think slips should be avoided with 40 flaps, and if slips are done make the slips very gentle not rough.

I think at altitude it would present a problem?

IIRC, flaps 40 and a slip could block the airflow over the elevator, causing an abrupt drop of the nose. No bueno close to the ground.

I am gonna call myth-busters on this idea of a nose drop for these older Cessnas with the 40 flaps. I have never seen any proof of it. Tail blanking does occur, where turbulent or dead air causes light buffeting as the turbulent air from the flaps bumps around on the tail a little bit. But this is not the same as tail stall. It might be possible with some combination of conditions like forward CG, gross weight, crosswinds, and aggressive maneuvering, but I have never heard of any accidents from that cause.

That said, there are plenty of good reasons to avoid the 40 flaps on electric models. Go-arounds are difficult, there is a risk of wing stall due and mis-trim given complete flap retraction, and you can get extra drag for steep approaches by forward slipping. The older models with the jack bar were ok because you could change flap settings quickly on those models. Too bad they gave that feature up to get more interior room.

I can confirm the tail blanking in a 172N model with 40 degrees flap, definately an experience.

I had a right side flap cable break on a C310, but couldn't confirm until I was on the ground. I did however think initially it was an engine failure because of the rolling and no other changes, then as I was performing the 'recovery' it hit me what was happening, I retracted the flaps and she flew just fine.

Agreed. In fact, I seem to recall slipping with F40 was never a limitation, just a "not recommended" maneuver. The reason being that it would create buffeting. I did them many times with students, and never experienced a tail stall or anything abrupt like that. If that were even a possibility, it would be a very well known limitation.

Even the new cessna R and S models have that placard next to the flaps, and they only have 30 degrees of flaps

Agreed. You aren't supposed to slip for long with low gas in one tank, because you might run out of gas, but I've done lots and lots of approaches with 40 degrees flaps and slips. I can say that at least once I did feel what seemed like "tail buffeting", but per the relevant manual, there was no loss of control or other ill effects. If this was a limitation...well, it would be a limitation...