A Microsoft file hosting and synchronization service.
In my case, I sync all my data with OneDrive, including things like template paths (Office) and project files (Scrivener). Moving all of that to a different directory would have a major impact on the applications using those paths. And my experience with OneDrive resync is that it takes days to resync everything, and OneDrive will attempt to resync everything after a move as described. So the only way I'd dare do this would be while keeping the sync with my local system disabled, and after the move-index-move, I still expect that OneDrive will attempt a full local resync.