star wars - In the new canon what do we actually know about Darth Plagueis?

I read and loved Darth Plagueis but with all the recent changes to what is and isn't canon I'm losing track.

There's a common theory that Supreme Leader Snoke is Darth Plagueis, most are discounted because Plagueis is a Muun. But where does that information come from? What current canon information do we have about Plagueis that disproves this theory and what has been marked as Legends?

Answer

The character of Darth Plagueis is only mentioned in two canon sources: Episode III and the novel Tarkin.

The only information from Episode III is that

Darth Plagueis was a Dark Lord of the Sith, so powerful and so wise he could use the Force to influence the midi-chlorians to create life ... He had such a knowledge of the dark side that he could even keep the ones he cared about from dying.

...

He became so powerful ... the only thing he was afraid of was losing his power, which eventually, of course, he did. Unfortunately, he taught his apprentice everything he knew, then his apprentice killed him in his sleep.

_{Darth Sidious/Palpatine (source)}

A little more information about Plagueis is given in Tarkin. Perhaps most important is the fact that Plagueis was indeed Sidious' master.

Save for Sidious, no sentient being in close to five thousand years had set foot in the shrine [under the former Jedi Temple]. The room’s excavation and restoration had been carried out by machines under the supervision of 11-4D. Even Vader was unaware of the shrine’s existence. But it was here that they would one day work together the way Sidious and Plagueis had to coax from the dark side its final secrets. In the intervening years he had actually come to appreciate Plagueis for the planner and prophet he had been. Such perilous machinations required two Sith, one to serve as bait for the dark side, the other to be the vessel. Success would grant them the power to harness the full powers of the dark side, and allow them to rule for ten thousand years.

_{p. 101}

Elsewhere it is mentioned that the droid 11-4D was owned by Plagueis.

Plagueis was still alive by the time Sidious became Naboo's senator, which is consistent with the Legends novel Darth Plagueis:

[Sidious and Tarkin] had met several years after Sidious -- still an apprentice of Darth Plagueis at the time -- had been appointed Naboo’s representative to the Republic Senate.

_{p. 102}

Plagueis on the Force:

Darth Plagueis had once remarked that “the Force can strike back.” The death of a star didn’t necessarily curtail its light, and indeed Sidious could see evidence of that sometimes even in Vader—the barest flicker of persistent light.

_{p. 102}

Plagueis disdained "trivial" political matters and did not foresee that Sidious would rise to become Emperor:

The Emperor spent a long moment studying Ison and Rancit, stretching out with his powers to discern alignments, configurations, some syzygy of events. Then his thoughts turned to Vader and Tarkin. He appreciated how well they were working together, but he began to wonder if they were perhaps too close to the details of the dissidents’ scheme to recognize their ultimate objective. One needed to have a safe remove, as he felt he had, gazing into the 3-D representation of the galaxy he had made his own. How Plagueis would have mocked him for allowing himself to become personally involved in such a seemingly trivial matter; but then his Master had never foreseen that his onetime apprentice would become Emperor.

_{p. 103}

That's it. That's all we know about Plagueis strictly from canon. We don't know Plagueis' species. The main proof that Plagueis is not Snoke comes out-of-universe, from Pable Hidalgo. In-universe, the best evidence is that Plagueis needs to have died in order for the prophecy of the Chosen One to be true (the Chosen One was supposed to destroy the Sith); Anakin is the Chosen One, so he couldn't have destroyed the Sith if Snoke is Plagueis.