"Unhittable" is a bit of a misnomer. It doesn't mean that you won't take any damage, but rather no hit will go unmitigated in some fashion.

There actually was a brief time, back at the end of TBC, where unhittable was possible. It was a terrible, terrible choice, because it only applied to physical attacks (and did not take Sunwell Radiance into account), but that was part of the reason Diminishing Returns was introduced. A tank who can just stand in front of a physical-only boss and never need heals is a problem.

Don't know if you're be able to hold aggro well, but I remember a video in TBC of a rogue tanking Gruul. He stacked so much agility that with raid buffs (and boss debuffs) he was 100% unhittable. He and like 4 others downed Gruul and just ignored his enrage mechanic of hitting harder and harder and harder because the rogue never got hit.

It will be interesting to see with reforging if it's going to be possible to reach 100% unhittable again. Probably with giving bosses expertise it will not be on latest content, but would not be terribly surprising to me to see a tank with a reforged outfit that's 100% avoidance.