Just to clarify. The attack speed calculation with EF works as follows:
APS = (WEAPON_BASE * WEAPON_IAS + ECHOING_FURY) * (1 + ADDITIVE_IAS)
And yes, this is a legacy affix that has been purged - some of the oldschool guys might remember the “+X.X attacks per second” affix on the original vanilla Echoing Fury. As one can deduct from the calculation, the only multiplicative %IAS multiplier in the game is the local “%increased attack speed” on a weapon (it’s local, because it only applies on one weapon and doesn’t have an effect for dual wielding). As of now Echoing Fury is additive to the Weapon Base, which is unique on its own; it essentially means the EF performs better for slower weapons and worse for faster weapons.
Personally, I advocated for them to change the IAS buff to be within the generic ADDITIVE_IAS category. We’ll see what they end up doing.