Simply navigate to your desired friend, right click their name, and select "edit" note. This will give you a prompt allowing you to classify your friend anyway you want and will show your note when you hover over the "notepad" icon near their name
Curious to why Riot hasn't made an announcement about this seemingly long lost feature? Volibar commented, via reddit, on why they haven't said anything yet:
"We didn't announce it because it's not on everywhere. This is one of those features that required way more work than you'd think, just due to the number of players that we have.
We'll be rolling it out to other regions over the coming days / weeks just to make sure that the service remains stable.
Enjoy, and if you have feedback, let us know as always!"