add deprecation message to Series.apply

Author: topper-123

pandas/core/series.py

@@ -4397,6 +4397,12 @@ def apply(
         Can be ufunc (a NumPy function that applies to the entire Series)
         or a Python function that only works on single values.
 
+        .. deprecated:: 2.1.0
+
+            If the output from ``func`` is a listlike of ``Series`` the output,
+            wrapping the output in a ``DataFrame`` instead of a ``Series`` has been
+            deprecated.
+
         Parameters
         ----------
         func : function