I am going to quote something from a recent Ask Riot and will put a link if you want to read the whole thing. I hope it will be useful for you.:
** We want One For All to come back. We like One For All. Unfortunately, the custom voting mechanism in champ select that let’s you vote on the champs—and effectively facilitates the whole mode—needs to be re-built by hand to work in the new client. We haven’t been able to get to this yet, due to the other things we’ve been working on (Star Guardian Invasion, yay!).
And honestly, it sucks. : ( I want to play One For All with the new champs (Xayah FEATHER STORM!!). Hexakill is in the same boat (we need to rework multiple UI’s to squeeze in the 6th player). We want both of these modes back, but don’t have a satisfying answer for when they’ll be back yet. I’m pretty confident they won’t appear by the end of 2017, so I’m hoping we can get to them next year! <3**
https://nexus.leagueoflegends.com/en-pl/2017/11/ask-riot-one-for-all/