What is the source of the prohibition for women to wear slits (if there is a prohibition)?
The basic Halacha mandates skirt-lengths covering one's knees. Ankle-length skirts, are obviously more modest. The issue of the slits has to do with the fashion industry's desire for women to be "suggestive" of her body form by "daring" the male viewer to "peek" here and there. This kind of behavior is contrary to tzne'ut morals, and would most probably constitute a problem of "lifnei i'ver, lo titein mich'shol".