The real question is whether Crawford said anything, since they've both agreed he's the final arbiter. I imagine it will show up in their FAQ or a new Sage Advice column. I think I tweeted the barkskin question to him awhile ago, but I never saw an answer. I imagine it's hard to keep up with all the questions.
Not to say I don't appreciate Mearls' responses. Even if they sometimes deviate from the final 'official' ruling, it gives me a gauge on the 'will it break anything' meter. So, whether you include shield or not in the 'flowchart' probably isn't a huge deal so long as everyone at the table is on the same page.
AD
Sure. But folks often ask what either of them have said. And of all the voices that have weighed in on the subject here or elsewhere, his is the most (and so far only) 'official' response on this specific subject.
For me the real question is what I, and my players, think. Sure, we like to know what the official rules say, and any clarification from official sources is helpful too. But in the end, we decide what we feel needs a bit of tweaking. We use at least 90% of the rules as is.
For us it's simple: shields, magical items, and circumstantial bonuses work because that's how it's always worked in my campaign. Dex bonus worked as well, but we're comfortable forgoing that now because the base AC that is provided by the spell now is higher than the +2 of the older spells.
The fact that Mike's tweet provides some support for that position is just a nice bonus. One of the things that was always a part of D&D (good and/or bad depending on your perspective) is that the DM, and to some degree the players, get to interpret the rules for what's appropriate for them. Part of this for us (since I've had such long-running campaigns) is how to deal with a new edition of the game, if at all.
For this particular spell, the answer for us is 'not much.' Druids could use a shield to get an additional bonus before, and they still can now. I don't see any reason why an additional temporary 18 AC is going to break the game any more than a 16 AC.
If that doesn't work for your campaign, or if it's ultimately not the ruling used in organized play, so be it.
Ilbranteloth