Current behavior
As documented in Atlassian Support, in Jira Cloud it is expected that users who have been deactivated or removed from the site can still appear in the backlog’s user quick filters and avatar list when there are issues assigned to them.

Reference:
“How to remove inactive users from the backlog?”

How to remove inactive users from the backlog? - Jira and Jira Service Management - Atlassian Support
Currently, the only way to remove these users from the backlog UI is to:

• Run a search like assignee in (inactiveUsers()) or assignee = <userId>
• Bulk-edit and reassign all of their issues to an active user
• Refresh the backlog

 
Requested change / feature:

Introduce a way to hide inactive / removed users from the backlog assignee avatars and quick filters without requiring reassignment of their issues.