When working, I had several midis and maxis. I always wore a slip with them, either a half slip or full dress. I also always wore spanx, and the skirts were of opaque material, but I felt more confident and "smoothed" with a slip. And you never can tell when standing in bright sunlight (as a young, newly engaged Lady Diana learned)! I was born in 1958, so grew up with slips!
They still exist - I have found them at Macy's, Vermont Country Store, and various online lingerie sites. Even pants liners are still out there.