From Diablo Wiki
Legacy items are Legendary and Set items spawned prior to v1.04. In that patch Blizzard greatly upgraded the stats of most Legendary items in the game, essentially creating two versions of every Legendary item. In almost every case the older item is less-desirable, but there are a few exceptions.
- See other Reference terms.
Legacy Item Details
Legacy set items will not provide partial or full set bonuses when mixed with current set items. Legacy set items will only mix with other Legacy set items, and the same goes for current Set items. In most cases the Legacy set bonuses were smaller than the current ones.
Blizzard's website does not list Legacy item stats. You can see them in the Diablonut database though, as every Set and Legendary item lists its values in previous patches, as well as the current game.