The step and handle kit was an optional extra on the strutted Cessnas
My bad. I've never seen one without it. I didn't know it's optional.
Best to stick with the placards.
I agree that you should avoid it, just like the placard says, but note that the placard says to avoid it; it's not prohibited/restricted. There are some situations when it's useful. I don't think anyone's ever actually loss-control because of the weird airflow effects.
As for slipping causing ASI errors, yes, but every slip does. And they're fun. There's no reason to avoid slipping in general.