If we return all parameters for liblinear in _train_kwargs which is most clean, then there will be much duplication of code in subclass, like _get_bias got called in every subclass.

We can also only return( or not return) parameters that need special treatment, which I think is what you suggest. But I think it is dirty to treat parameters unequally in the first place.

Of cause, current implementation is also dirty in the same sense. And return class_weight_ in _transform_data is dirty too.