Hey folks :)
We have enabled the new datatype for identifiers now. New properties will start showing up at https://www.wikidata.org/wiki/Special:ListProperties/external-id. As announced we will also start converting existing properties with datatype string that should be external identifiers. We will start with the first eleven from https://www.wikidata.org/wiki/User:Addshore/Identifiers/0. We will do more over the coming days. There are still a lot of properties to go through so please help with that so we can get this over with quickly to cause minimal disruption to 3rd parties relying on our data. You will also see the identifiers that are already converted moving to a new section at the bottom of the item pages. This might need a purge still though to clear the cache.
Cheers Lydia