Morgana mid is strong (53% WR), support is balanced (50% WR), 9.9 hit support harder. Here's why.
Her base shield on Black Shield has gone from 70/140/210/280/350 to 60/120/180/240/300. The AP ratio was completely untouched.
Morgana mid almost always maxes E last on top of the fact that she still has the AP scaling on E. Morgana mid keeps her AP ratio and only loses like 10 shield overall throughout most of the game.
Morgana support often maxes it second or even first in certain matchups. She also buys less or no AP, instead relying on base shield, Forbidden idol line, or utility items for the bulk of her shielding. This change makes Morg support lose, proportionately speaking, significantly more shielding aside from the first few minutes of the game than Morgana mid does.
Why would you choose to nerf the base value over the ratio when the richer, AP building Morgana mid is the problem? This nerf changes almost nothing for her while making Morgana support's life significantly harder against mage supports.