This is a new feature. Easy enough to do. And once pinned in macOS, the note stays pinned in iOS.
To pin or unpin a note in iOS, swipe the note to the right. You’ll see a pin/unpin button. Nice.
How to pin a note to the top of your list in macOS High Sierra
To pin a note in iOS 11, swipe from left to right on its entry in the “index”; the left-hand column in landscape mode on iPad.
Hold down the right mouse button or Control and the left mouse button and choose “Pin Note” from the drop down menu.