commands: index: Warn users that this is an experimental command

Author: Noah Gorny

src/pip/_internal/commands/index.py

@@ -49,6 +49,12 @@ def run(self, options, args):
             "versions": self.get_available_package_versions,
         }
 
+        logger.warning(
+            "pip index is currently an experimental command. "
+            "It may be removed/changed in a future release "
+            "without prior warning."
+        )
+
         # Determine action
         if not args or args[0] not in handlers:
             logger.error(