Yeah, the only thing without doubt is that this is the last we'll see Michelle Gomez in the role, and even that I wouldn't consider a guarantee, she could always come back if they do another Master crossover in the future.
But in no way was Missy's death a permanent end for the Master. Assuming the Simm Master was being truthful when he said he used the full effect of the laser screwdriver on her and that this was sufficient enough to prevent regeneration there are still plenty of options explaining how she could survive. For all we know, Simm was lying, or more likely just as Missy remembered herself telling Simm to always carry a TARDIS component, maybe she had access to enough memories to remember to bring along some protection. Perhaps, just enough so that, yes, the laser screwdriver is still lethal, but negated enough to allow regeneration.
Regardless, all we see is Missy lying on the ground and her eyes closing. Considering the Master has survived at least two cremations, not to mention being tossed into volcanic pits, stomped on by dinosaurs, vaporized by Daleks, swallowed by the TARDIS engines, and dozens others, anyone who seriously thinks Missy's death is truly the end for the Master really doesn't understand the show.
We'll get a new Master someday. That's a guarantee. Probably even without an explanation, which is something that's only started in the new era anyway. In the classic era the Master's survival of apparent death was always brushed off.
"I thought you died when we last met?"
"Obviously not."
Hell, the Master's ability to survive death has even been lampshaded a few times. The Doctor's narration to the TV movie refers to the Master saying "even in death, I could not trust him." Also, in Utopia, upon learning that Yana was a Time Lord, the Doctor seemed to instantly work out he was the Master.
