Sorry for posting on this very old thread, but it looks like we are using "Backport to X.Y" labels to also backport PRs and is needed as a trigger for the automation (miss-islington) to work.
I am not sure if this automation existed when this label was retired, but it looks useful to me now.
Yes, there are a very few PRs with security fixes that come up and can be manually backported, but I wonder if it hurts having the label around?
thanks, Abhilash