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

Microsoft has taken down the Version 1809 Update:

https://support.microsoft.com/en-us/help/4464619/windows-10-update-history

As I wrote here in the first post and reiterated yesterday, there is possibly more involved here than the new OneDrive Folder Protection feature being set  to sync Documents, Pictures and/or Desktop to OneDrive by default.  That's on the AutoSave tab in OneDrive settings.

On the next tab over under Settings, there is also a choice to save space on smaller hard drives by having the "Files on Demand" stored only in the cloud and no longer on the hard drive.  If this is selected they are removed from the hard drive to the cloud and all that is left is sym-links to download them. 

Having moved my User folders into OneDrive for years, I know that during Version Upgrades the moved folder locations can be reset to default.  It's possible they are then sync'd as deleted and moved from Onedrive to the Recycle Bin.  I started noticing after 1803 that these were being set as default in some cases.

There is another issue with syncing the Desktop folder to a location off the hard drive that causes a fatal loop at startup that we're beginning to see more often now. 

Back up your files to external metal/flash now via a manual and not automated method (MS Backup utilities lose files too!), add a secondary cloud backup like the more reliable Google Drive Backup which let's you choose any folders to sync up to 15gb free to the cloud.

In previous catastrophic cases I've triaged (including Digital Activation collapse after RTM release) we had CSS in the main tracking thread pulling cases.  I don't see them here, nor do I see any cause yet rising to the top as most likely suspect.

The update has been pulled, so no more issues with relocated/deleted data. Hope those affected can recover ok.

It's a OneDrive feature that is triggered with the 1809 update, and the Documents folder seems to be checked by default. You can see this by going to Settings in your OneDrive app, and then the tab Auto Save.

If that's what's causing the deletion of users' files, I sure hope Microsoft engineers are targeting and reversing those erroneous settings, and will make OneDrive retain the current settings when updating.

You can test it quite easily by going to the OneDrive settings. If you have Explorer open while enabling the settings you will see that the respective folder will be created under the OneDrive folder structure, and you can see the original Desktop/Pictures/Documents folders empty and then repopulate under OneDrive.

Actually, what is happening is listed in the following article:

https://support.office.com/en-us/article/sync-your-documents-pictures-and-desktop-folders-with-onedrive-d61a7930-a6fb-4b95-b28a-6552e77c3057?omkt=en-US&ui=en-US&rs=en-US&ad=US

It's a OneDrive feature that is triggered with the 1809 update, and the Documents folder seems to be checked by default. You can see this by going to Settings in your OneDrive app, and then the tab Auto Save.

Clicking on Update folders will give you the option to protect important folders and if you activate it, the folders will be moved to OneDrive to also enable roaming between devices.