The whole issue of slits is overblown in my opinion. if the slit is WELL below the knee, then the part of the leg that we are mechuyavos to cover, is already covered. Personally, I take more notice when I see women with seamed stockings, which IMMEDIATELY draws the attention of the eye to it. Normal people do not look at slit skirts and get excited. If the slit was up her thigh, THAT I could understand. But frankly, I think too much has been made of this issue, and there are more important tznius inyanim that people should concern themselves with. I’m with Pashuteh Yid on this one. Be happy the women are not wearing short skirts or pants. Slit skirts are more comfortable to sit and walk in. Period. If the rabbanim would suddenly say there is no problem with them, no one would give a second thought to it being untzniusdig. It is how we are conditioned to think by the rabbinical interpretation of some people, that causes us to think certain things are not proper. Not all people agree with this interpretation.
To answer the question of where it says in the Torah what we have to cover – that is in the Torah sheh b’al peh. It doesn’t say in the written Torah how to wear tefillin, or shecht an animal, either – – but Moshe Rabbeinu got the exact info from Hashem at Har Sinai and that mesorah was passed down from him to yehoshua, and so on down the ages until Rabbi Yehudah Hanasi codified the Mishnah out of fear that most people would forget the Oral Law. I would like to know where in the Gemarah it discusses slit skirts as assur, however. I am not being facetious, I really want to know. I might rethink my position.