Would slits in a skirt be Assur because of דת יהודית?
Correct.
The following is a quote from the Minchas Yitzchok:
We are alarmed to hear of a new dreadful breach in the wall of tzniut in relation to female dress. The fire has already caught hold of the periphery of the camp of shomrei Torah umitzvos. This is to wear dresses that have slits either at their sides or in the center, which is a manner of dress that represents pritzus and rebellion. A Bas Yisroel who wears such a garment transgresses Das Yehudis and encroaches on the cardinal issur of Giluy arayot.
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