In the final episodes of Season 10 World Enough and Time and The Doctor Falls we find out that after the events of The End of Time, John Simm's The Master was pulled back into Gallifrey with Rassilon the other Time Lords.
Following this the Timelords fixed The Master's decaying body as thanks for helping stop Rassilon's plans as according to the Wikia
Upon getting back to Gallifrey, the Time Lords, grateful for the Master's help in preventing Rassilon from executing The Final Sanction, properly restored his body. With that done, the Time Lords kicked him off Gallifrey. However, the Master stole a TARDIS to go back into the universe; so he considers it a mutual being kicked out.
Source: The Doctor Falls (TV Story) > Plot (3rd Paragraph)
given the mention of "back into the universe" this would indicate that The Master's recovery was after the events of The Day of the Doctor after Gallifrey was removed from the universe rather than being destroyed by The Moment.
after this The Master arrived on the Mondasian colony ship and lived like a king until he was overthrown
After that, the Master landed on the colony ship, lived like a king and killed people at his leisure. However, the colonists eventually overthrew him and the Master attempted to flee. However, as his TARDIS was on the last level, too close to the event horizon, it burned out his dematerialisation circuit and left him stranded. "You never could drive could you?" the Doctor asks.
Source: The Doctor Falls (TV Story) > Plot (3rd Paragraph)
this is when time become wibbly wobbly as after some time Michelle Gomez's The Master (here-by referred to as Missy), who is the next form on John Simm's The Master, showed up. in The Doctor Falls Missy remembers when she was The Master on the colony ship being yelled at by "an angry woman" and carries a replacement dematerialisation circuit.
following this and the apparent betrayal of The Doctor Missy stabs The Master thus showing that this was the start of his Regeneration into Missy however as Missy leave The Master shoots her in a back and prevents her generation. as such The Master, now wounded and will begin regenerating, descend back down to his Tardis while Missy is dead.
Does this mean The Master is dead with Missy being the last? while it's unknown if The Master will return with the new writer for the series after Steven Moffat, but we do know that despite being fully dead The Master has been resurrected multiple times, the most recent resurrection was what lead to The Master's decaying state in The End of Time
so even if Missy never regenerated The Master could return for any reason and considering The Doctor said in Hell Bent
death is Time Lord for man flu
The Master's current status (as of 02/03/2018) would be sick with man flu until further notice