A Microsoft file hosting and synchronization service.
As of this morning (Sat 5/04/2025) my shared OneDrive folders are accessible again in Windows Explorer on my Windows 11 laptop. The folders appear slightly different, with a blue chain link icon in the lower left corner, and moving files from my personal OneDrive to one of the shared folders is considerably slower than it was in the past (takes about 15 to 30 seconds now), but at least it's working again like it used to.
Files in shared folders that had previously been opened and cached locally are gone, though, and all shared folders are online only again, with files being downloaded as needed when I access them.
Looks like for me the wait is over, and I'm back to the way it was. Hope it will straighten itself out for all of you as well ... eventually. Why this migration (or whatever it actually was behind the scenes) wasn't communicated to us remains a mystery, though.