What does xgb's scale_pos_weight parameter do for regression?
There are a few unused or deprecated parameters in both XGBClassifier and XGBRegressor, so it might just be a matter of sloppy inheritance/c+p.

A couple of possibilities:


They copied over the params from Sklearn's GradientBoostedClassifier
They copied over the params from XGBClassifier
They inherited the properties from some class that already had those attributes.


There have been some consistency issues for a while now (I faintly recall nthreads versus n_job issue.)