windows 10 (October 2018 update) (version 1809) deleted all my files

Blaming the customer for the failure of the vendor to do their job (install a feature release/do not destroy data) is not acceptable.  If a backup is a key element, then shouldn't said vendor have an install step that ensures that a backup is made before continuing?  Just wondering as well.

Again with the false dichotomy.  No one is blaming the customer for Microsoft's failure, just as no one blames hardware manufacturers when hard drives fail, or when motherboards fail, or when anything else catastrophic happens to a computer.  Microsoft's failure in this instance could be mitigated without the failure being considered the user's fault.  This is the opportunity to remind everyone why a conscientious backup process is important, regardless of the source of failure. And regardless of the source of failure, data protection is the user's responsibility.

Microsoft has been unusually quiet about this, probably because of the disastrous effect it could have on customer trust in the cloud which is a big part of their plan for the future. 

Business and enterprise customers may be moving in that direction, but I don't think the majority of Home users have any interest in Cloud file storage, as evidenced by the many opinions expressed in these forums.

In my case, there was no Windows.old directory. I've searched the entire C: drive for the missing directories and files using both the Windows Explorer search and Recuva, and I've checked OneDrive, all to no avail. MS claims to have the tools to help us recover and urges us to contact them (https://support.microsoft.com/en-us/help/4464619/windows-10-update-history) which I did on 10/6. I was given a case # and an appointment time for MS to call me back on 10/7. I received an email about this appointment, yet no one called and there has been no contact since then. How long am I supposed to "minimize use of the affected device"? I have important time-sensitive work that needs to get done!

When/If MSFT every does call you back, may be good to ask them what they're doing with the PB worth of personal data they're collecting from this debacle :-|   Can't help but to think some developer, in his/her infinite wisdom, decided to plant an Easter egg that backfired, big time!  Shame on you, Microsoft!!

At this point I would run File Recovery software like Recuva.  http://www.piriform.com/recuva/features

It's important not to do anything else as if files are deleted they are normally still there but hidden and only have their references in the partition table removed.  If nothing is written over them they can have these restored to be recovered. 

Microsoft has been unusually quiet about this, probably because of the disastrous effect it could have on customer trust in the cloud which is a big part of their plan for the future.  Losing files is the worst imaginable PR, and it's leaking out that they were warned about this issue by previewers for months before going ahead and releasing it without fix. 

Latest is that they claim to have a way to recover files if one contacts Support but those calling Support find they know nothing about it:   Microsoft Says They Can Recover Files Deleted by Windows 10 October 2018 Update

As a front-line MVP who has triaged these catastrophic issues for years - including the near-collapse of Digital Activation in the RTM after it was the only major feature not stress-tested - what I miss here in the tracking threads is the team of a venerable CSS manager Charles Erwin who handled triaging these issues back to XP and retired recently. Normally they would be in the thread working with us to sort causes, pull logs and assure customers.  We have heard and see nothing except the media reports and talking to each other.  Possible causes remain what I wrote in the first post.

Very helpful, thank you!