This caveat is buried deeply enough in the macOS Big Sur release notes that a lot of people are going to be bit by it. Creating a new volume in an existing APFS container had become the de facto best way to install a second OS.

&10;Screenshot of text from release notes: "If macOS Big Sur 11 beta is installed into the same APFS container as previous versions of macOS, system software updates can no longer be installed on the previous versions of macOS. (64411484)"