This is because the Notes app will be gaining some new features in iOS 15 and macOS 12. By upgrading to the beta of either software, this means that if you were to create a new set of Notes on your device, a device running an older iOS or macOS build might not be able to access it due to compatibility issues.
For example, if you upgrade to iOS 15 beta but your Mac is still running on macOS 11, then the notes you create on your iPhone might not be visible on your Mac. You will also be greeted by an alert/error message that says that the note has been hidden on some devices. This isn’t that big of a deal, but it might be something to think about if you rely on the syncing of your Notes app quite a bit.
Both iOS 15 and macOS 12 are expected to be released later this year, possibly during the fall, so maybe waiting for the stable release might be a good idea.