SharePoint Online Site Deletion blocked by Retention Policy although no content is retained

Thats expected. Containers under the hold (eDiscovery or Retention Policies) even if the retention period has passed/expired still can't be deleted. Containers need to be excluded from the holds/retention policies > return success before it they can be deleted.

Sometimes you'll have an invalid retention policies too (most commonly caused by a deleted retention policy that was scoping All for the specific location/workload).