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

Sorry for the loss of your VM file. Not only does it exceed the capacity of a normal OneDrive, but it breaks this rule...

https://support.office.com/en-us/article/sync-your-documents-pictures-and-desktop-folders-with-onedrive-d61a7930-a6fb-4b95-b28a-6552e77c3057

Sync your Documents, Pictures, and Desktop folders with OneDrive

"File exceeds the maximum file size: OneDrive can't sync files over 20 GB. Remove these files from the folder you want to protect and then try again."

Looks like 1809 set it as default to protect the contents of those folders. But the feature couldn't handle it when a rule was broken. And your VM file now swims with the fishes.

It sound like a Kafkaesque data proteciton racket, I never asked for OneDrive to begin with. But anyhow I am genuinely happy that they pulled the update and that prosperity won't be talking of the Microsoft great October Document Purge (MODP). If they did roll it out over 150 or so million clients with a high percentage of affected systems then the amount of lost work would be mind boggling. Its probably a lot more then the hours spend on building the pyramids and going to the moon put together. Pulling it certainly avoided some bankruptcies and might even prevented some suicides. 

I hope this worst case "near hit" will be study material for OS developers and will galvanized the patch testing processes.

Whew! I feel extremely lucky that I switched to Google Drive (out of cheapness probably -- 15 free GB and "unlimited" photo storage) and moved my needed/cared-about files there from OneDrive. Another salient difference between GD and OD is that GD doesn't strongly insist on a JunctionPoint/SimDir for MyDocuments, Pictures, Music, etc. 

I feel that with that move (including turning OD sync off) I probably spared myself Robert Z's misery, and to add my 2 cents to this discussion: Backup is of course a great idea, but backing up Ghosted files ("On Demand" as they are called by OneDrive for a couple years now)  is a bit of a minefield with almost all software (I use Acronis 2019 and haven't tested it with 1803 -> 1809 updates).  I'm certain many Millennials on the OneDrive team weren't on the original Groove team from 2005/6 when MSFT made that acquisition, and could never have foreseen the "cloud-first" principal carrying to such an extreme such as the 1809 debacle implies, nor seeing Groove tightly integrated with the Operating System 10 years later. 

Good luck to all who are impacted, and please consider waiting for Feature Updates (as opposed to Security Updates) to give them time to "bake" as so many different MS teams seem to be involved with the former, and testing is a bit more "rigid" on the latter. Again my 2 cents as an 'ex-Softie' :)

"Are you quite sure your regrettable data loss has nothing to do with your own failure to ensure safe and backup on thumb drive, USB-drive, NAS, whatever?"

That's blaming the customer.  Patches shouldn't hurt.  Period.  

totally agree here.

Hindsight is always 20/20, too bad we all cant go back and have a do-over whenever we want. It doesn't help to chastise people who have lost their documents, they probably wont let it happen again. The blame falls squarely on microsoft's shoulders. sorry

The blame for the initial data loss is indeed on Microsoft, no doubt about it.  Users are responsible for protecting their own data, however, from whatever calamities might occur.

I have been perusing the tech journals, and noticing that Microsoft seems to be pushing the testing back on the Insider, and will undoubtedly employ "go slow/slow-ring" QC on future updates. The point missed, however, is this: Feature Updates, however regular (monthly, quarterly, semi-annually), are just that: Features, not core O/S functionality patches. My vote would be to make the "optionality" of FUs (or in this case set the 'FU bar' lower ;) a leetle more visible, for those IT admins who lack time for full "Insider" regression testing every six months. Going to this extreme shouldn't be a Reg Hack or even a System Center Configuration Manager push to the enterprise to slow the flow of Feature Updates (up to 365 days).

Also, speaking for rural folks that get their internet from a dish and are metered or throttled every six months during download of a 4 GB OS "pull" by several unknowing machines in the household, it is a very real imposition, even if I don't lose any data, to watch the latest "Better Call Saul" in jerky Low Def.

If they must be mandatory, clearly just setting a bit flag for severity amongst "Insiders" will not be a silver bullet, and use of the "free resource" of Insiders by Microsoft may be a concept worth reconsidering. Let's face it: This isn't the first time in Microsoft history that a "cool feature" ended up causing a whole lot of bad days for IT pros and end users alike!

That is really very bad behaviour for such a update.

Microsoft does not seem to be able to deliver updates bug free. 

Please make feature updates  voluntary!!

They do in their little known LTSB version. Its a version of Windows  without the extra store and cortana junk and only brings security updates instead of "feature updates"

LTSB is a version of Windows 10 Enterprise, not available to the general public.