If anyone else is using the feature, here is the reply I received from Github Support:
The ability to act a user for your integration is one of the features provided by the Early Access period:
The ability to access this feature for Integrations was removed by our team on February 10th, 2017. We’re sorry for any confusion that this may have caused as you were developing wi
This feature includes the following API endpoints:
POST /installations/:installation_id/access_tokens with the user_id parameter
We have decided that this is not the best solution for acting as a user for an Integration and we will providing an alternative method in the next few months.
I guess this endpoint was being used by both the “impersonation” and “user identification” features, and when “impersonation” went away - so did this.