We should mention somehow the deprecation of _safe_tags in estimator checks in the 0.23 release notes. Although it's an internal change, it can lead to common checks for an estimator breaking in 0.23 with confusing error messages (e.g. tslearn-team/tslearn#231),

def check_estimators_dtypes(name, estimator_orig):
[...]
>       if estimator_orig._get_tags()['binary_only']:
E       KeyError: 'binary_only'

Those errors are symptomatic of estimator tags not working properly in contrib packages e.g. because the mixins / BaseEstimator need to be re-ordered to follow MRO and make est._get_tags work. Although that change happened in 0.22 users haven't noticed it, because due to _sage_tags things were working even with incorrect MRO.