Warn when meta is passed to apply (#10256)

Author: phofl

dask/dataframe/core.py

@@ -4330,6 +4330,15 @@ def apply(self, func, convert_dtype=no_default, meta=no_default, args=(), **kwds
                     M.apply, self._meta_nonempty, func, args=args, udf=True, **kwds
                 )
             warnings.warn(meta_warning(meta))
+        elif PANDAS_GT_210:
+            test_meta = make_meta(meta)
+            if is_dataframe_like(test_meta):
+                warnings.warn(
+                    "Returning a DataFrame from Series.apply when the supplied function "
+                    "returns a Series is deprecated and will be removed in a future version.",
+                    FutureWarning,
+                    stacklevel=2,
+                )
 
         return map_partitions(methods.apply, self, func, args=args, meta=meta, **kwds)