We recently (#10483) moved sklearn.preprocessing.Imputer to sklearn.impute.SimpleImputer to make way for other imputation strategies (and to acknowledge that imputation is a kind of learning problem in itself).

We also want to get rid of the axis parameter. In fact we did this in #10558, before realising that if we're deprecating all of preprocessing.Imputer, it's unnecessary. Since SimpleImputer is not yet released, we do not need to deprecate the parameter.

So your mission is to emulate #10558 but don't bother with deprecation:

• to remove all mentions of SimpleImputer's axis
• to update the what's new entry about moving Imputer to SimpleImputer with a comment that axis has been removed