> How does #2460 will handle binary case? 

As it is at the moment, I haven't change the logic around the positive label handling.

>  Will it still return one value for "positive" class or return ROC for both classes (i.e. no need in pos_label in this case)?

It detects if y_true and y_score are in multilabel-indicator format. In that case, there isn't any ambiguity on the number of classes. The format checking can be easily done by checking the number of dimension of y_true/y_score. It doesn't handle the problematic multiclass task. 

Depending on the chosen averaging option, you will get one value for the all binary tasks or one for each task.