Issue Summary

Currently, there's no Rest API that can retrieve a list of teams that aren't assigned to any issues, or otherwise aren't "used". 

Using the current Teams GUI only displays issue content from the site you are viewing the team page from.

Suggestions

Assuming the user is an org admin:

1. Create an API endpoint that allows org admins to get a list of teams that don't have any issues assigned to it
2. Create another endpoint that allows org admins to get a list of teams that have fewer than two members
3. Create another endpoint that allows org admins to list teams by their last activity date

Workaround

Assuming you are an org admin and have access to all of your sites, you can use the Team activity info box to determine if a team is actively being used based on the last update.