There was an ask riot about this a while back
http://nexus.leagueoflegends.com/2017/07/poro-king-again/
We still aim to build new modes and release them as often as we can to you guys (we just released Blood Moon and Dark Star modes this year). Unfortunately, we can’t build a whole Dark Star mode every month though. In between, rather than have nothing at all on the weekends, we bring back some reliable favourites (EG: Ascension & Poro King), so there’s still something fun to play when you want a break from the regular game. We feel like this is better than the alternative of nothing at all to play in between each new big mode. For those of you who remember, we actually used to run game modes that way pre-RGM, and it felt pretty quiet for the long stretch between each mode (~3 months at a time). This way, there’s still some fun stuff around on the weekends to play with, even if it is something that’s been seen before.
We’ve also been steadily building new client compatibility for the more successful older RGM modes (One For All, Hexakill), but #gamedev means it’s slow going sometimes. We’re doing this in parallel with building our new modes (EG: Blood Moon & Dark Star). The good news is, we now have the Doom Bots of Doom stood up in new client! Newer modes are also dev-freebies with being built in the new client from scratch, so they are much easier to bring back (Blood Moon just went live and Dark Star is coming up again soon).
TLDR: Most game modes are not possible on the new client yet.