To OP: Agreed, that is quite frustrating change ...
But i believe i was pissed about it in previous patches aswell (i wanted to sneak inside after i joined Minthara) ... so it must be older date.
Underdark is fully lit also.
Rly? O_o
I was sucesfully sneaking through Underdark in most places ... so i would dare to say that there is enough shadows for you to use.
But many objects there are luminiscent ... for example that mushrooms, that looks like vertical tubes ... so you need to be more careful about that.