We're going through and making adjustments based on this thread, other feedback, and our own internal testing. Some of the issues were pure oversights; others reflect a loophole when it comes to self-inflicted or friendly-fire damage, for which we're working on a comprehensive fix.
I do want to clarify, however, that our changes are only intended to offset the direct effects of the "squish" and our goal has been to make sure that linearizing the item curve between item levels ~60 and 463 doesn't impact the ability to solo legacy content. That is not the same as saying that we guarantee nothing will change, period - we've made a number of adjustments to tank mitigation, healing, defensive cooldowns, and so forth. That's nothing new for an expansion, and in some cases, yes, that will make old content more challenging than it was in 5.4.8, especially for tank specs that could previously use Vengeance to great effect. But that has nothing to do with the item squish.
There is no specific intent or plan for when and whether anything becomes soloable. That's up to you.
We design the content to be appropriately difficult for the original audience at the appropriate level (whether that's a few players in the outdoor world, a dungeon group with a dedicated tank and healer, a full raid group, etc.). Over time, as player power grows through access to higher levels, more powerful gear, new talents, and so forth, content that used to require a group becomes soloable, organically. Some classes (tanks, those with better self-healing, those with pets) will be able to do it sooner than others, but everyone will get there eventually. It might take an extra tier, or even an extra expansion, but that's just the nature of how the power curve works. And if you can't solo something but are interested in a shot at a transmog item, achievement, or rare mount, you can always bring a friend or two.
We don't nerf old content specifically to make it soloable - we occasionally make quality-of-life changes when a mechanic (like Thorim's cleanup that reset the fight if the arena was unoccupied) is artificially preventing players who have the raw power to solo an encounter from doing so. But we don't go back and specifically make old content numerically easier. .
