To check whether a Team has Projects in it, go to Settings > Workspace Features > Teams.
In the Teams Settings menu, Click Edit Team/Projects for the Team you need to delete. This menu will show any remaining Projects in the Team (including those that have been archived). You'll need to either delete or move these Projects before proceeding.
Once all Projects have been moved or deleted, click on the trash can icon next to the Update Team button. You'll be asked to confirm that you want to delete the Team; if so, click OK.
You'll see a pop-up confirming that the Team has been deleted, and it will disappear from the Teams list.