Оk, here are my findings all. Took me 2 days but I think I figured it out.
Mobs that were added to the game in classic are dropping loot that is not scaled. Mobs added post-WoTLK are dropping scaled loot. Haven't found any TBC / WoTLK added mobs to test those 2 expansions, so not sure where exactly the line of scaling is placed. This is not intended by any means, so moving this thread to the Royal League may be a thing to consider.
Edit: Scaled gear has (min zone lvl - 1) ilvl and (min zone lvl - 4) lvl req.
What that means:
- low lvl characters can farm a few mogs from the rares added post WoTLK, which do drop scaled items (ilvl 19, lvl req 16) in 15-60 and 20-60 zones.
- low lvl characters can farm ilvl 24-25 green BoEs, which do drop from Classic rares in the 15-60 and 20-60 zones. Those items are better than dungeon drops and in most cases BiS for waist/gloves slots.
- (theory) any lvl characters can farm low lvl blue BoEs from Classic added mobs in dungeons. (If there are any pre TBC mobs in SFK, there is a good chance they will be dropping the unscaled ilvl 24 version of SF, because their loot table should work just like the chest's once)