Test plan and sharepoint site group

Hi Catalin and welcome, When you deleted your plan the group should also have deleted. Please see this brief support article by MS https://support.office.com/en-gb/article/Delete-a-task-or-plan-39e10e78-13f0-446d-94cd-9e562648497a If you deleted the plan and then immediately went to the Group, it could take a few minutes for the group delete process to happen. If it has not deleted after 5 mins or so, go to the Group click on the ellipsis icon, select 'Edit Details, and at the bottom of the screen see if you have the Delete Group function. I would suggest that if the Group has not auto deleted that you submit a support ticket as I am sure MS would like to know. I hope this helps.