Immutable storage for Azure Blob storage enables users to store business-critical data objects in a WORM (Write Once, Read Many) state.
Immutable storage supports:
Time-based retention policy support: Users can set policies to store data for a specified interval. When a time- based retention policy is set, blobs can be created and read, but not modified or deleted. After the retention period has expired, blobs can be deleted but not overwritten.
Reference:
https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blob-immutable-storage