Nobody: a kick pleat or insert may work.
Funny, 30 years ago I was taught that slits are not assur per se but just to realize that wherever the slit begins, consider the skirt that short. So a slit situated such that the knee would still be well covered when sitting was no problem. Somewhere along the line, slits became assur altogether in my circles. If I ever run into the teacher who taught me the first approach, I’d love to know what happened. Meanwhile, I have no problem toeing the line.