deprecate old_mode values

old_mode was introduced as a temporary way to disable some of the new behavior, until the old applications are updated with it. It's not stored per SP and not replicated.

This means every non-default value of old_mode is by design automatically deprecated. It should behave as such — setting it should issue a deprecation warning. And the usual deprecation rules (remove when all versions where it's not deprecated are EOLed) should apply.

This task is about printing a deprecation warning when a old_mode is set to a non-default value.