Find a Jira Cloud user's accountId and assign, reassign, or unassign an issue (post-GDPR identifier rules)

Documented steps

1. Understand the identifier rule: username and userKey were removed by Atlassian's GDPR/user-privacy migration. accountId is the only stable identifier accepted by the API.
2. General user lookup: GET /rest/api/3/user/search?query=alice — query matches displayName and emailAddress by prefix. Read 'accountId' from each result.
3. Better for assignment: GET /rest/api/3/user/assignable/search?project=ABC&query=alice (or use issueKey=ABC-123 instead of project). This returns only users who can actually be assigned in that context.
4. Assign the issue: PUT /rest/api/3/issue/{issueIdOrKey}/assignee with Content-Type: application/json and body {"accountId":"5b10ac8d82e05b22cc7d4ef5"}. Success is 204 No Content.
5. To unassign, send {"accountId": null}. To hand the issue to the project's default assignee, send {"accountId": "-1"}. These are two different outcomes — null leaves it Unassigned.
6. You can also set assignee inside a create or edit payload as fields.assignee = {"accountId":"..."}, which is cheaper than a second call when creating.
7. Guard your matching: search results are display-name based, so verify you matched exactly one user before assigning, and prefer assignable/search so you never assign someone who lacks project access.