As the title says, champion mastery is now available to earn in ARAM & TT up to level 5. \>o</
As with RGM modes, levels 6 & 7 will not be available. For more context around this you can see a post from Riot Socrates here:
I'll keep an eye on this thread so, if you guys notice any bugs please let us know in here! Cheers PBE peeps.