"Execute as" no longer available as an option for Incoming Authentication under OAuth

Summary

The "Execute as" option is no longer available for Incoming Authentication under OAuth.
This is required in order to use the Knowledge base functionality between JIRA Service Desk and Confluence, without having to expose the knowledge base space to anonymous viewing.

This comment describes such functionality having become available, however this does not appear to be possible anymore in 5.10.x versions of Confluence.

Steps to Reproduce

1. Set up an application link between JIRA (Service Desk 3.1.x) and Confluence 5.10.x
2. On Confluence end, navigate to the application link settings and change the incoming authentication.

Expected Results

An "execute as" option should be available from the dropdown men

Actual Results

The expected option is not available.

Notes

This option is vital for users that have a Service Desk environment, and want to make use of the knowledge base functionality.
In certain use cases, it is not possible to expose the content of the knowledge base publicly, which rules out the option to allow anonymous viewing of the space(s) in question.

The functionality has been confirmed to work with a 3.1.x Service Desk environment connected to a 5.9.x Confluence environment.
As such, a customer upgrading to Confluence 5.10.x could potentially see knowledge base functionality break.