Wired
Apple has expanded the functionality of the “Memo” app on iOS 15 and macOS 12, but it has become clear that memos created on the latest OS may not be viewable on older versions of the OS.
Two new features not available in older OS versions
The Notes app on iOS 15 and macOS 12 adds the ability for users to create tags to help them sort notes quickly and easily, as well as a “name mention” feature that makes important updates for shared note members. You can now notify each other about. You can also view the recent history of shared notes with the all-new Activity View.
However, the “Create Tag” and “Name Mention” features are not compatible with older OS versions of the app, so it detects that your iCloud account is using iOS 14.5 or macOS Big Sur 11.3 or earlier. If you do, you will be warned that the two features are not available.
Source: 9to5Mac, Apple
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Source: iPhone Mania