Folder /Library/SystemMigration

Yes, I have it too. Evidently stuff quarantined from the private and System folders of the previous install of OS X in case they had been compromised in ways that SIP now protects those folders from.

From the point of view of that folder being vestigal, it does seem to be an oversight that they are included under SIP protection as it is just a waste of 803.3 MB.

But I guess the reason it's contents are preserved in the first place is in case some may have had important files in there. Perhaps the folder is intentionally protected to make it clear to anyone trying to re-instate the contents of those folders, that they are re-introducing elements that potentially compromise SIP.

-Max.