I think it would be good if we could determine whether a model requires y in fitting in a programmatic way. This came up in https://github.com/scikit-learn/scikit-learn/pull/16112/files#r378281885 but it also relevant for AutoML systems or generally for interacting with many estimators in a programmatic way.

This is somewhat related to the _estimator_type. I think we should have a new tag is_supervised or maybe supervised for this.