In the past, Google's Android beta program has supported some non-Pixel devices as well. This time around, at launch, the beta program only supports Pixel 2 and newer devices. However, Google does note in its announcement that the Android 11 beta program will support other non-Pixel devices "in the coming weeks" as well.
OPPO has already confirmed to XDA Developers that it will be releasing Android 11 beta builds for the Find X2 and Find X2 Pro later this month. Other OEMs are yet to make an announcement, but if the last Android beta programs are anything to go by, selected devices from OnePlus, Nokia, Sony, Asus, and Vivo should also be a part of the program. These companies should reveal the list of compatible devices in the coming weeks.
The next beta of Android 11 is scheduled to be released in July, with the final release on track for a Q3 release. We should ideally see some Android OEMs join the beta program before the second beta of the OS drops next month.
If you own a Pixel phone, you can join the Android 11 beta program by following this guide.