I can only find old posts about Data Center on this subject. The custom field will become null.
But in cloud, I notice it will display the deleted user as "former user". How can I search for such values? i.e. customfield_10100 = deletedUser()
What happens to the user field value when the user gets deleted? Does it become EMPTY or something else?
I cannot find any relevant functions in this page:
https://support.atlassian.com/jira-software-cloud/docs/jql-functions/