According to the 2/20/2020 Community Update,
- February: Mythic Rumble replaces default Free-For-All in February - March 5: Mythic Arena rotates into Ranked for MarchMythic Arena has replaced Mythic Rumble, and therefore has replaced the only FFA mode found in Halo 5. I was fine with regular FFA being replaced by Mythic Rumble as I assumed regular FFA would be rotated back in once it left.
I hope 343i brings back a FFA playlist of some sort.