Deprecate default for series

Author: jseabold

pandas/core/groupby/grouper.py

@@ -509,6 +509,7 @@ def __init__(
 
                 if observed is None:
                     warnings.warn(_observed_msg, FutureWarning, stacklevel=5)
+                    observed = False
 
                 self.grouper, self.all_grouper = recode_for_groupby(
                     self.grouper, self.sort, observed

pandas/core/series.py

@@ -1673,7 +1673,7 @@ def groupby(
         sort: bool = True,
         group_keys: bool = True,
         squeeze: bool = no_default,
-        observed: bool = False,
+        observed: Optional[bool] = None,
         dropna: bool = True,
     ) -> "SeriesGroupBy":
         from pandas.core.groupby.generic import SeriesGroupBy