Users that have installed the betas of iOS 15, iPadOS 15, and macOS 12 Monterey have likely tried out some of the new features offered by those betas’ new Notes app. New features include support for tags and @ user mentions in shared notes.
You may have also discovered the notes use these two new features don’t show up in Notes in some earlier versions of iOS and macOS. Users with devices still running older devices running older versions of the operating systems should avoid the new features.
If the Notes app detects a device in your iCloud account running a version earlier than iOS 14.5 or macOS Big Sur 11.3, it will alert you that tagged notes or mentions will be unavailable on those devices. Interestingly, the latest versions of iOS 14 and macOS Big Sur are partially compatible with these features.
If your other devices are fully up-to-date and running iOS 14.5 or macOS Big Sur 11.3 or later, then it appears that any notes using @ mentions or tags are able to be opened by those versions.
As usual, MacTrast and Apple both warn users to not install any betas on their daily driver iPhone, iPad, Mac, or other devices. Instead, only install betas on devices set aside for testing purposes.