Riot had said in very early days of LoL (se1 iirc) that there may come a day when they could retire champions. Most likely due to them having too much identity into something that's unhealthy for gameplay. (EG: super long term/60 second stealth...old Twitch...they fixed that with completely overhalling stealth/vision control instead.)
Though I think they went more with full rework method after that. So unsure if the early statement is still true.
I'd say it's possible they'd still delete a champ, but not from being underplayed. More overplayed and very unfun to play against, and no nerfbatting or changes properly fix them.
Some of the heavy reworks were pretty much deleting a champ and adding a new on in their place. (Sion)