How do I return to my old home screen with my folders

I don’t either. I never use the App Library; years ago I created my own logical arrangement of my 460 apps that is much more useful and easier. There are a few iOS features I like, but I don’t consider them earth-shaking.

I’m happy you got your Home Screen back.

I had the same experience. I was just checking out the new widgets and suddenly all my app folders were gone and I had page after page of apps in alphabetical order. A web search found lots of people on developer forums having the same experience with various iOS 14 betas. Clearly, the problem persists in the public release.

Your old home screen should still be there; the app library is just one page that you can make the last page and never use it. That’s what I do.

If you lost your folder organization that should only have happened if you went to Settings/General/Reset - Reset Home Screen Layout or Reset All Settings.

I was already restoring my iPhone from a backup. I expect it to take 2-3 hours.

To reset a phone to its default settings you need to drill down a couple of levels in the Settings app and then confirm that it's what you really want to do. OP and I were both on the home screen rearranging apps & widgets when it happened spontaneously. So ...

There's a bug in iOS14—you can read a little more about it here: https://developer.apple.com/forums/thread/656888.

I imagine there'll be more of these posts as people upgrade.

Just restore your most recent backup. That happens if the phone is reset to default settings.