ok, it's been a while since the arena is out, but as the mode is removed, they should remove the Relative Archievements too, only for the one you have not completed. is not my fault if i back in time i have not finded the way to complete them at all. the mode isn't playable because they have shut her out, but as the mode, they have to remove the objectives, cause now is LITTERALLY IMPOSSIBLE to complete them, oer bring the mode back ( i am pretty sure that now it could be very populated)
"but fed, if someone have done some objective, will they despawn?" no, for the people who have unlocked them all, they have to stay, but the people that haven't they don't have to worry about seeing the "reach the first place 100 times" locked at the 76%...
