Custom formatting of tokens for date fields In D8.1, I have created a field called field_date and added it to a content type.

I try to use pathauto to try to configure a URL pattern for the content type like type/year/title. But the token browser does not expose expandable date options for date_field. If I enter it manually, I get "The Path pattern is using the following invalid tokens: [node:field_datetime:custom:Y]."

How can I format the output of a token for a date field, the same way I can for a node property like date created? 

I've searched the issue queues for contrib token, core datetime and core token system, but I find nothing about this.
The Token module added direct support for datetime and daterange fields in December 2016. See https://www.drupal.org/project/token/issues/2786181. 

You should be able to use something like [node:field_datetime:date:custom:Y]