Apple is keeping the list of supported iPhone and iPad models the same as for iOS 13. Here's the complete list:
You'll have to factory reset your iPhone or iPad and either restore it from the last backup you created when it was running iOS 13, or start fresh.
I suggest creating an encrypted backup using Finder on a Mac or iTunes on a PC right before you install the beta. That way you have a recent backup should there be too many bugs or issues for you to deal with.
There was much more news coming out of Apple last month, so make sure to read through our complete WWDC 20 coverage.