Should shield blocking be effective against magic damage?

[Lyth]

You can mitigate A1S on PLD with Sentinel, Hallowed, Sentinel. You can throw in Rampart to mitigate a few cleaves along the way. The main issue with this in a progression situation was needing to wait for Hallowed to come off cooldown after a wipe, but that's a non-issue now. There were no limitations on your ability to tank this on PLD, and interestingly enough, the world first clear was done using PLD/WAR.

Unlike Sheltron, DADM is not free. It costs you DA, and by extension, dps. It's also the reason why Sentinel is objectively more powerful than Shadow Wall. Interestingly, if I choose not to spend the DA cost, the combination of Shadow Wall (30% DR) and DM (15% DR) gives me the same damage reduction as Sentinel on magical attacks (0.7 x 0.85 = 0.595, which is about 40% DR). Whether I pay that extra cost will depend on whether it gives me more uptime out of stance, which in turn depends on gear and the amount of damage I take. There are a few ways that I could do this. I could go SW+DM, LD, SW+DM. Alternatively, I could go SW, LD, SW and use DM and Shadowskin to eat cleaves. It's not that I'm mitigating something that a PLD cannot. The extra cooldowns just give you more freedom out of stance.

Reprisal and DD aren't used in isolation to mitigate tankbusters, and are always used as adjuncts to other abilites. On average, PLD block procs on their own account for an average damage reduction of about 6% over the other two tanks in a physical fight, before taking RoH into account (there was a reddit thread earlier this summer which collected the data on this). Reprisal is 10% damage reduction with a theoretical maximum uptime of 66% (in practice, this will be less), which on average just about offsets the passive contribution from block procs in single target.

In both physical and magical fights, WAR has access to ToB. This isn't quite as good as Rampart on a single hit tankbuster, so you would likely pair this with IB, but it's still a versatile option.

Sheltron is free, and is not stance locked. It mitigates more damage in a single hit than IB. If this extended to magical attacks as well, it would be incredibly unbalanced. You probably wouldn't require any additional cooldowns on most fights.