The other maps
When it was released, we were promised a ranked queue on Crystal Scar. Years later, that has not happened.
Champion mastery is not awarded for maps other than Rift.
Magma chamber is just scrapped, I guess.
Some champions have mechanics that cannot be used on other maps (Azir passive on dominion, Morde R passive everywhere but rift)
A lot of these would take some time to implement, so I can forgive them, but no champion mastery? Seriously?