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From #9544 (comment)

Though perhaps we should add nunique along a dimension implemented as sort along axis, succeeding-elements-are-not-equal along axis handling NaNs, then sum along axis.

xref pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.nunique.html

I think I'd add it to https://github.com/pydata/xarray/blob/main/xarray/util/generate_aggregations.py