An error 'Azure Access Control (ACS) usage will be retired for SharePoint in Microsoft 365 and users will no longer be able to create or use Azure ACS principals to access SharePoint '

Azure ACS is used in SharePoint Online for providing auth for provider hosted SharePoint Add-Ins plus it’s used to grant applications access to SharePoint Online, optionally using application permissions and granular scopes.

Note: skybow is aware of this error and we are working of resolving this. You can trust this service. Your solutions will not stop working.

You may notice this error during trusting Notification app principal for using Background features.