Just logged into the PTR to see if they affected the legacy seasonal Bindings of the Lesser Gods. Usually a legacy item retains their legacy property since they have a unique item identifier. For new updated item properties, you would normally have to get it in a new release as a drop, rendering the older item legacy.
Not the case here. Seems the removal of the “Split Fire Allies gain 5 times this bonus” is global to the entire game, including the transferred seasonal gear (now legacy). As OP and broken as that bonus was, it would have been nice to retain it as a Seasonal artifact and fun to play (like a bonus for those who played the season). Similar to the days when the legacy Cold dmg Frostburns could still co-exist uniquely after a seasonal upgrade and could be worn at the same time as their newer incarnation (cubed) . As it was mentioned in the PTR release notes, this is Just a heads up on the status of the legacy Bindings for those wondering. Game on.