I believe that mages are now at two extremes when it comes to their damage. Compared to season 13 split 2, mages have an easier time killing squishy champs but a way harder time killing tanks.
There is more flat magic pen and ap and less health on mage items, which I think is good. However, this flat magic pen mainly helps against squishy targets. And then there are overtuned items like storm surge that are mainly strong against squishy targets.
However, season 14 item changes make mages terrible against tanks. Demonic embrace and liandry’s anguish being gone really hurts. Now the only max health damage mage item is liandry’s torment, which has a weaker max health burn compared to liandry’s anguish. Also, the new liandry’s torment does not have the old agony passive, which provided 0% − 12% bonus magic damage based on target’s bonus health. This helped out a lot with killing tanks. There is nothing like this in the new set of items for mages. In the new season, tanks also have new or buffed mr items, like force of nature providing more mr. Tanks are already hard to kill, but it feels almost impossible for most mages with these new items.
I’m afraid Riot will nerf mage items to lessen their damage against squishy champs, with no compensation damage buffs against tanks. Then mages will go back to the same way they were before. Compared to other champs, they will be good against squishy targets but will be terrible against an enemy team that stacks any health and magic resist. Maybe void staff can get a passive similar to lord doms to help mages out with killing tanks.
It still depends on the mage you are playing. A Ryze will still be able to dps a tank down and a Lux has no business killing tanks anyway.