Today (01 October 2018) we have published details about the Jira Cloud REST API changes related to user privacy improvements. This also marks the beginning of the deprecation period, ending on 29 March 2019. During this time, API methods will continue to support existing behaviors (i.e., ability to query by username/userkey, receive responses with username/userkey, etc.;); however, at the end of the period, those capabilities will no longer be available.
Jira Cloud REST API migration guide and deprecation notice: https://developer.atlassian.com/cloud/jira/platform/deprecation-notice-user-privacy-api-migration-guide/
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