There was no theft involved. Chief Justice Tyrest appropriated the armor of the original Ultra Magnus, which have been worn by loadbearers prior to my use.

[They have loadbearers in Starscream's timeline, right? He wonders where their universes diverged if they also had the Great War but Ultra Magnus was Optimus Prime's spark brother. Perhaps someone had manipulated the hotspots, leading to this difference.]

He was killed in battle, if you are wondering how he came to his end.