Darth Plagueis is dead. He has been dead since Darth Sidious killed him, rising to the rank of master in the Sith, as told in 2005's Star Wars: Episode III - Revenge of the Sith.
That's the bottom line from Pablo Hidalgo, of Lucasfilm's Star Wars Story Group and continuity king. Hidalgo has authored multiple books like visual guides and encyclopaedias about Star Wars in addition to being one of the members of the team who help keep the worlds of comics, books, tv, and movies together.
Hidalgo is also nearly constantly harangued on twitter by fans asking questions about canon and continuity. While he's previously said things like "Snoke is Snoke," a conversation over the weekend saw him get very specific about the theory that Darth Plagueis, the master of Darth Sidious aka Emperor Palpatine somehow survived and eventually became Supreme Leader Snoke in Star Wars: The Force Awakens.
"His apprentice killed him," Hidalgo said of Plagueis. When pressed, he said, further, "Sidious killed Plagueis. Ascending to master. As is the way of the Sith." and "I said Sidious killed Plagueis. He killed him. Killed. As in "to kill". Like, there was killing."
Believe it or not, a fan actually thought that was still too oblique.
"He is dead. There you go. Three words," Hidalgo said, clearly exasperated.0comments
So yes, the dead character is still dead. Supreme Leader Snoke's identity is not a hidden link to Palpatine's past. Now quick, everyone start bugging Hidalgo with your other theories. He'll be thrilled.
Maybe we'll learn more about Supreme Leader Snoke from the second Aftermath novel or Star Wars: Episode VIII.