There's some breaking changes that we wanted to do for a long time but didn't do for 1.0 but I figure it might be worth starting a list somewhere for 2.0, since they keep coming up and we might forget them.

• in OneHotEncoder change handle_unknown to warn by default (and maybe recommend target encoder or max_categories or something)
• Make train_test_split assume the second entry is y and stratify if the estimator is a classifier, like all other cv methods do. Maybe call the attributes X and y and allow arbitrary additional entries.
• Make Pipeline do a clone in __init__.