TLDR: It feels bad to practice in ranked, but normal games are too laid back making it hard to properly practice your league gameplay.
I don't know if other people have had the same experience as I have had or not, but I feel as though there is no appropriate place to practice (SR). Obviously you would not want to take a champ that is brand new for you into ranked, so you look towards normal games for practice. The problem with normal games though, is that most of the players in your normal games are going to be very uncompetitive, meaning that at a certain point in your LoL career you are going to just stomp every normal game no matter what champion you are on. This has purely been something I have noticed lately, and kind of a deterrent for me. A real example of this for me is Zoe. Lately I have been having major trouble hitting her E, which tanks my mmr/rank. If I were to pick Zoe in a normal game however, even missing E as I would in ranked I might go 17/6 or something around there.