The "Radio Select" custom field has this option, but it's no longer an option for the "Dropdown (single)" custom field, and by default, any user that can set that field value can also add anything to the list, regardless of whether their permissions permit access to Custom Fields. In my case, this field is "Lost Reason" for Opportunities, and every typo and random addition is being added, and it's creating chaos.

Please bring back the checkbox and/or the ability to turn it off again to align with permissions.