Actually, modern Who has been providing explanations for the Master's return. We got exposition about his role in the Time War in The Sound of Drums, and TEOT showed him being resurrected. Granted, at first when Missy showed up we didn't really get much of an explanation of how the Simm Master overcame his degenerative Skeletor problems, though one was sort of provided when Simmy showed up in season 10. We did get an explanation in The Witch's Familiar of how Missy survived Cyber-Brigadier shooting her though....which they've already done with The Master.