Trailbreaker and Silverbolt completed a mind transfer, allowing them to slip past their personalized cell security devices and release the other prisoners held in the Cauldron, a dimension within Unicron. Saber and countless others were able to escape. Saber presumably returned to his own dimension. Universe #2
Dan Khanna tried to slip Saber into BotCon/OTFCC comics for years, but Glen Hallit threatened that if he ever did, he'd write his death into the next issue. Khanna apparently lucked out hiding him in amongst the big escape scene; Saber does not die in Universe #3.