Hi,
The source server enters a maintenance state where they still contain the same files they always owned (never delete the files from the source server) but are not available to users and apps. You can then stop using these servers at your convenience.
There is no specific statement as to whether "Delta-DATA" will be taken care of.
For an introduction to the storage migration service, you can refer to this document:
https://learn.microsoft.com/zh-cn/windows-server/storage/storage-migration-service/overview
Best Regards,
Wesley Li