The process of building custom applications and tools that interact with Microsoft SharePoint, including SharePoint Online in Microsoft 365.
I had the same read-only problem and after dealing with different people at Microsoft what actually helped me unlock the site is in this page: https://learn.microsoft.com/en-ca/troubleshoot/sharepoint/sites/site-is-read-only. Just click the "Run Tests: Locked Site" and follow the instructions. You need admin access for this of course.
And for the September 23rd date even though I pay for that subscription every month, I have no idea what´s going to happen.