In October 2018 we announced that user names were deprecated, in line with the removal of user names across all our products. See https://developer.atlassian.com/cloud/jira/platform/deprecation-notice-user-privacy-api-migration-guide/ for more information.
Going forward Jira cloud will remove support for turning wiki text containing user names into user mentions.
Old unsupported wiki text user mention format (with user names):
Current wiki text user mention format (with Atlassian Account IDs):
(Note that the wiki editor in Jira already writes user mentions using Atlassian Account IDs for you.)