I've never worn a slip under a skirt.
I remember trying on my mum's half slip, when I was a small child, which she never wore as she lived in jeans and trousers, most of the time and had absolutely fantastic, long, slim, legs; so had no real need of one, even when she did, occasionally, wear one of her lightweight, cotton, Laura Ashley skirts.
I don't like wearing see-through clothing (too flimsy), or long skirts in general (too constricting) - like my mum, I prefer trousers/leggings (on the rare occasion I have worn a skirt, in the last couple of years, I've worn a mini with opaque tights), so have no real need for a slip, either.
I did buy a plain black satin slip dress, back in the mid '90s, but when I tried it on at home, my mother looked absolutely horrified and said I looked like I was wearing underwear (which was the whole idea, of course, but nevertheless, I was very young and impressionable and trusted my mum's judgment, so it put me off!), so I never wore it (much to my BF's disappointment!).