In the last few episodes of Gundam Seed, at the battle of Jachin Due. Yzak and the Duel fought and destroyed Shani Andras' Forbidden by sacrificing the Duel's assault shroud pack as a decoy. Yzak then moved on to help out Dearka Elsman and the Buster by defeating Clotho Buerer's Raider, using the Buster's rifle
In the final moments the Buster was severely damaged by Rau Le Creuset's new mobile suit, Providence, it could be assumed that the Buster was damaged beyond repair.
Both gundams may likely have been decommissioned, off-screen, at the end of the war.