In October 2018 we announced that user names (and user keys for webhooks) 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.
While that deprecation period is now over, as of today user names (and user keys) are still returned in webhooks. For usages that do not use Atlassian Connect, user names are still supported in Jira Cloud REST APIs. (User names stopped working for Connect around April 2019).
Starting next week (Monday December 16) we will begin the process of removing user name support completely from Jira Cloud webhooks and Jira Cloud REST APIs (in line with the privacy migration guide linked above).