It would be very difficult to code losing depravity levels in game, especially with the depravity skills. I also didn't expect very many people to lose enough depravity for it to be very important. So for those two reasons it's not really planned. Lowering your depravity does prevent you from gaining any more levels unless you regain all the depravity you lost though.
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Yeah, that kind of sucks for my low-depravity run, though, since I got a level by accident because I didn't replace my miniskirt with something more modest before... either the bra or the cursed heels. Still, there are ways to lower the weakness effect level through purity skills, so that's something.