Schnorr protocol is witness-hiding?
I found the answer:
Schnorr protocol is witness-hiding - proved in GGM model in https://www.shoup.net/papers/dlbounds1.pdf (as part of proof that Schnorr identification protocol is secure).

At the same time, it's not possible to prove it in standard model based on discrete-log problem: http://www.cs.cornell.edu/~rafael/papers/schnorr.pdf