I think all of them should be left behind, not just a select few. Why? It's Y2. Y1 weapons should be able to handle the beginning of TTK activities same as they do right now and as you progress through TTK they should become less powerful - which in turn makes you desire getting new weapons to make the content easier - and finally that they basically are completely useless in end game content. So if you bust out your Gally on Oryx the hit marker should read immune. And yes, when Y3 hits, all of Y2 items should also become obsolete in the same manner. Of course Destiny already screwed up this entire natural progression by allowing for Fatebringer, example, to be ascended to 365 and now they're allowing some of the older exotics to be viable.