AtlasLoot does not use Ace2; it does, however, use Dewdrop, which requires AceLibrary from Ace2.
For the most part, though, taint doesn't matter. Taint is only a problem if you get "action blocked" messages. Since you're using BugSack, just enable "Filter addon mistakes" and you won't be shown taint messages. If you get "action blocked", then it can be helpful to check taint messages, but otherwise they're not really something you need to worry about.
AtlasLoot has AtlasLootFu built-in, and AtlasLootFu downloads Ace2. I put up a suggestion on the author's site to separate those 2 in the future, since AL uses Ace3 and ALF uses Ace2.
For most addons, no, there is nothing in Ace2 that would prevent authors from migrating to Ace3. Mostly it's just author laziness ("Ace2 still works so I don't want to spend my time rewriting my addon to not use it"), lack of time ("I don't have much time and since Ace2 still works I'd rather spend the time I do have on more fun/productive things"), or simply the fact that the addon is no longer maintained.
For a few addons, there is the problem of AceOO, which has no non-Ace2 equivalent and makes "upgrading" addons that depend on it a major undertaking.
Which specific addons do you have that use Ace2? I have several, and have never had any problems.
I have the Curse Client install libraries separate from addons, so I can't tell you quickly what mods I use that still rely on Ace2. I could come up with a list at some point.
I know, for one, AtlasLoot still uses Ace2. I believe ArkInventory also still relies on Ace2.
I don't know what exactly I was doing when I got this Ace2 error. I had BugSack installed so I am able to at least track LUA/taint errors easily.
Is there a known solution? If so, does it involve a lot of recoding or is it a quick, one-line fix?
Unfortunately, a couple of very popular mods I use still rely on Ace2. Is it up to the author of these mods then to move everything to Ace3? Is there something in Ace2 that isn't in Ace3 that would prevent mod authors from moving to Ace3 entirely?