Lothar’s Sword was last seen on Draenor wielded by Turalyon before the Planet exploded and as was already mentioned in this Thread the Official Audio Book A Thousand Years of War states that the Lightforged Draenei forged Turalyon’s current Sword.
Why it was made to half look like a Sword of Light and half like a Broken Sword: to honor the broken Sword of Lothar and also have a more effective weapon to fight with.
Where Lothar’s Sword is now: Probably either in Turalyon’s personal shrine to Lothar or stolen by Deathwing and given to Nefarian.