@Bjørn Olav Sagen Welcome to Microsoft Q&A, Thank you for posting your here!!
Visual Studio Online plan will basically manage your development lifecycle in the DevOps and integrated Azure portal.
Check if you have created any project using Visual Studio or DevOps. If the resources in the resource group are deployed using Visual studio do not delete the plan.
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
-------------------------------------------------------------------------------------------------------------------
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.