upgraded iCloud notes
If you see an Upgrade button next to iCloud in the sidebar of the Notes app, you haven’t upgraded your notes yet. This means that until you upgrade, you can’t apply paragraph styles or checklists, browse the attachments in your notes, see and restore recently deleted notes, lock your notes, or add people to notes.
If you upgrade your notes, you can view and edit them on other devices with OS X 10.11 or later, iOS 9 or later, or iPadOS that are signed into iCloud using the same Apple ID. You can’t access upgraded notes on devices with earlier versions of OS X or iOS. Also, any notes you create or edit on those devices won’t appear on devices with upgraded notes.
When you upgrade, most of the formatting in your notes stays the same. However, any notes with HTML formatting (notes you created by copying and pasting from a website) may look significantly different.
See iCloud Support.