how to handle unassigned tickets on hold

Yes, it is possible! In Jira Service Management, you can have free users with limited roles. Simply:

Steps:
Create a new user, for example:

Name: retention.ticket@yourdomain.com

Display name: On Retention

Don't grant this user an agent license (no access to the project as an agent).

Give this user permission to be assigned (through project permissions):

In Project Settings > Permissions

Make sure that Assign Issues and Be Assigned include a group or role where they are (e.g. “Users”).

Now you can assign tickets to this user, even if they don't have an agent license.

✅ Result: they appear as “assignee”, but they don't consume license or interfere with the team's work.