We had two options
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We could just gut the shit out of Janna, then gut the shit out of Soraka, then gut the shit out of Sona.
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We could just nerf this overbuffed monstrosity of an item.

We're going to gut our bot lane waifus.
We could just gut the shit out of Janna, then gut the shit out of Soraka, then gut the shit out of Sona.
We could just nerf this overbuffed monstrosity of an item. 
We're going to gut our bot lane waifus.
:thinking: i'd rather just nerf all supports across the board
Hmm... after spending a couple hours stewing over this... I think Janna is being moved away from being a Shield Mage. She's already got the best kit for disengage, which is quite preserved in these iteration. Riot seems fine with that identity and wants to take away the Shield part of her kit. It's still there, sort of like how Orianna has a shield, but it's no longer meant to be her identity.
They've done nothing to the shield. It was the ability of hers they changed least besides her ultimate. Once you hit rank 5 it's literally the same as live.
I'm not saying you can't play her as a shield mage. You certainly can. You can also play Orianna and Lux as shield mages. I just feel like Riot is trying to discourage Janna from only being played as a shield mage.