Though legacy loot was implemented in Legion’s Dungeons and Raids, apparently only bosses were affected by the new loot rules as trash packs still drop loot as before.
Why is this such a big deal?
Well, in Nighthold, in every difficult, there are BoE cloaks that are unique transmogs, meaning you can only get them by looting trash, which barely drop anything.
In Antorus, there are many BoE transmogs that are unique and only drops from thrash.
But the real deal here is Tomb of Sargeras, there are many Tier Set items which are BoE and only drops from thrash, an example is the Monk set, which the belt and the boots, on all difficulties, only drops from thrash.
“Why don’t you just buy from the AH?” You might ask. Well, since trash isn’t dropping stuff following the legacy loot rules, those items are on an extreme lack of supply, and because they are heavily demanded since they are set pieces, the prices can get really high.
Please take consideration on this Blizzard, dungeons and raids from older expansions already have trash dropping items more often, and locking set pieces behind trash while those mobs barely drop anything make those sets unnecessarily hard to complete, this demotivate me a lot to do those transmog runs, even when with legacy loot on the bosses.