It is possible that this could have been leaked to more than just Pixel 4 owners, but based on a tweet from XDA's Mishaal Rahman, the users who received the software have a Pixel 4 XL. Which could mean it's an isolated case with just the Pixel 4 series phones.
And maybe just the Pixel 4 XL in general. In any case, the software is out there now on at least a few phones.
The update size is 736MB and comes with the build number RPB1.200504.018. Other than there doesn't seem to be any more information for the build. But Google did already lay out some of the details on what users could expect anyway.
Pixel devices getting the Android 11 beta probably wasn't planned
Some users have speculated in the tweet comments about the Android 11 beta software that sending it to these Pixel devices was planned.
While there's no way to prove that it wasn't, Google did clearly state that the launch was being postponed. It was originally planned for a release on June 3. But Google confirmed over the weekend that it would be waiting to send out the software due to recent current events.
What has likely happened is that these users were mistakenly sent the beta alongside the security patch. Which is also being sent out to a wide number of Pixel users today.
Google hasn't set another date for the official beta drop yet
Seeing as it's June 1 and the beta release and reveal was set for June 3, the new drop might be a little way's out now. Google has yet to set another date for the official drop of the beta.
Noting in the tweet that it would be sharing more on the release soon. Soon could mean anything from a few days to a week. And it's entirely possible that things were pushed to next week Wednesday. Which would be a full week from the originally intended drop date.
However, with at least two people already having the leaked beta on their phones, you might see it be made available unofficially before then. If any users who did get it choose to share the software.
If you're part of the Android beta program, you can always check to see if you get the software by mistake as well. Though it is worth mentioning that the users who reported getting it received it through an OTA update.
Oh god….Google messed up.
— Mishaal Rahman (@MishaalRahman) June 1, 2020