What does this even mean though?
If you mitigate 10% of the damage or 20% of the damage of incoming damage (a 100% increase in performance) amounts to something like a 12% difference in healing required on one tank. Even if up to half a raid's damage taken is tank healing, and a single tank's uptime is half the time... the difference between a mouth breaking tank and a professional tank can be made up for by a single healer having a 2-pc set or a few item level increases. Do you see how fucking broken that is as a system?
If healers don't meet the HPS requirement of the fight, the raid dies. If DPS don't meet the DPS requirement of the fight, the raid dies. If a tank is bad, healers have to heal very slightly more?
Fuck the old school, Wildstar style tanking system where tanks exist simply to hold threat and have enough HP to not be 1-shot by mechanics and where you only push buttons because you don't have anything else to do doing while you act as a living bob the punching bag. Fuck that system.
Since it may be too late in the development cycle for Blizzard to actually fix this shit, I can only hope that the tank drought is so severe that subscriptions are impacted to the point that the expansion after Legion sees a return to the current glorious era of tanking.