Why did they decide to make tanking gear obsolete in most important bosses, and how can i effectively tank tfb and soa to make up for this handicap imposed on my character? Is there a way to mitigate their dmg as a sith assassin?
The only boss fight in the game when tank gear is completely superfluous is Soa, and that's because all of his attacks are Force/Tech attacks (so they ignore defense and shield chance and only use Resistance chance, which can only be tweaked by talents and set bonuses, even then, only up to 2%) and deal Internal/Elemental damage (and so use only your I/E DR, which isn't affected by your armor at all). For Soa, the only thing you need a tank for is someone to be your perpetual taunt spammer and keep the attacks focused on a single person (who will then drag the boss around in p3).
For *every other boss fight* however, your tank gear matters. If you're taking massive amounts of F/T damage when fighting TfB, it's because you're fighting the boss on the outer ring of platforms rather than the inner ring (if you're on the outer ring, the boss wails on your with F/T attacks that I'm pretty sure are also I/E damage based; if you're on the inner ring, the boss just wails on you with M/R attacks that you can actually mitigate effectively). The only fight other than Soa that actually approaches the realm of "dear developers, my tank stats are worthless" when properly executed is Kephess the Undying, where most of the damage you can expect to take as a tank ends up being K/E F/T damage (which means that Shadows hate it and the other tanks just kind of shrug it off as normal).
The important caveat here is that, as a Shadow tank, you always have your self heals to fall back on from a mitigation standpoint. It might not be much but, if you're taking damage, your self heals are providing you with some degree of countermeasure against said incoming damage, even if nothing else is. Against Soa, Shadow tanks are actually the best, even if our stats are functionally worthless, because we've got the 2% resistance chance, the best I/E DR, and self heals that account for a pretty substantial portion of our total survivability (and self heals are *never* worthless from a mitigation standpoint). On Kephess the Undying, we take roughly 20% more damage than Guardians and 35% more than Vanguards, but we've got our self heals to counteract that additional damage taken (assuming you can regularly stand still in a non-doom-patched location for 3 straight seconds with Kephess close by every 15 seconds, which hasn't really been my experience, which is why I hate that fight so very, very much).
The developers have actually recognized that the first 2 operations they developed really sidelined the importance of tank mitigation stats (there were a lot of threads started and rants ranted at release about tank gear being largely pointless in the face of massive amounts of F/T attacks and I/E damage). Thankfully, EC and TfB have both done a pretty good job of ensuring that tank stats are important (Kephess the Undying excluded): EC and TfB have damage profiles that *vastly* favor M/R attacks, especially compared to KP and EV, assuming you do the fights properly. If you're feeling that your mitigation stats aren't important, I'd suggest you look at your strategy and revise it. There's not much you can do about Soa (not that it matters much since it's laughable content now), but TfB is definitely a fight where you're to blame for taking loads of F/T damage rather than M/R.