As of The Last Jedi release (12/2017), there was almost NO additional canon information that clarified who or what Snoke was. We see a better image of him in Episode VIII, we see details of his fashion sense, but that's about it - presumably, on purpose by Story group.
As the intended assumption is that he's now dead by the "hand" of Kylo Ren, chances are we wouldn't be finding those details out anyway. Let The Past Die.
Personally, I'm starting to fall into CBR's point of view camp: "Snoke May Be Star Wars’ Greatest MacGuffin".
 - except for a vague thing about Sages of Dwartii and obsidian ring from Mustafar, from Visual Dictionary. And an even vague maybe possible visual similarity between his face and outline of Prime Jedi in Visual Dictionary.
To complement Andy Serkis, Pablo Hidalgo on Twitter just confirmed that Snoke isn't Plagueis (or rather, that Plagueis "truly, deeply" died.
Witka @timvitkuske Apr 30
@pablohidalgo Can you confirm that Snoke is NOT Plagueis? Can you confirm that Plagueis is dead?
Pablo Hidalgo @pablohidalgo
@timvitkuske Plagueis was killed by his apprentice. He's dead.
And further in a later tweet:
Witka @timvitkuske May 8
@pablohidalgo my real question to put it to rest, is did Plagueis' spirit survive physical death through the dark side? @colliderjedicouncil
Pablo Hidalgo @pablohidalgo
@timvitkuske he was said to possess the ability to keep loved ones from dying. Not himself. His apprentice killed him.
... and just to be absolutely sure:
Pablo Hidalgo @pablohidalgo May 8
@Daniel_Outhier @Gary_Thursby @timvitkuske Sidious killed Plagueis. Ascending to master. As is the way of the Sith.
Marissa Talchi @MarissaTalchi
@pablohidalgo @Daniel_Outhier @Gary_Thursby @timvitkusk You didn't answer the Q. Is he actually dead or does Sidious think he's dead?
Pablo Hidalgo @pablohidalgo
@MarissaTalchi @Daniel_Outhier @Gary_Thursby I said Sidious killed Plagueis. He killed him. Killed. As in "to kill". Like, there was killing
4:10 PM - 8 May 2016
The actor behind Snoke (Andy Serkis) has just stated to Entertainment Weekly that Snoke is an original character
Although there was much speculation about who Snoke might turn out to be (one popular theory was that he’d turn out to be a Sith Lord known from the prequels as Darth Plagueis) he actually turns out to be … just Snoke.
Serkis said the character was aware of the events from the earlier films but was a new addition to the storyline.
Of course, he may have been mis-informed himself (ala secrets in production of OT), or misinterpreting something, or intentionally hiding the truth for Disney; so until we have independent iron-clad confirmation, this is convincing but not conclusive evidence.
Yes, there's some scant evidence. Frankly, I think the theory is false because Kylo Ren isn't a Sith (as per J.J. Abrams interview).
The evidence against contains statements from J.J. Abrams, Pablo Hidalgo and Lawrence Kasdan. Oh, and George Lucas's statement that "Chosen one bringing Balance to the Force" means that Anakin got rid of all Sith :)
The evidence for is based on the facts that:
In Legends/EU canon (Darth Plagueis book) Plagueis was a Muun. Muuns are:
Look like this:
Supreme Leader Snoke is:
Tall and thin.
But Neal Scanlan, chief of creature and droid effects, told PEOPLE, “This character is much better executed as a CGI character. That’s just a practical reality when he’s 7-foot-something tall; he’s very, very thin.” (source)
From Alan Dean Fosters TFA novelization, Chapter X:
Seated on the raised platform that was the focus of the chamber was the blue-tinted holo of Supreme Leader Snoke. Tall and gaunt, he was humanoid but not human.
Seated in shadow, the tall, slender form loomed over the other two men.
Looks slightly kind of like a Muun
UPDATE: having watched the film, he does look VERY MUCH like a picture of Muun from Clone Wars cartoon in certain frames. I will post screenshots when I can get them.
A Spanish language magazine posted a drawing depicting several characters, Snoke among them. There, he looks VERY MUCH like a Muun from Clone Wars:
News site Colliders published a video confirming this story.
Then, according to the article linked, LucasFilm asked Collider to remove that picture (that looks exactly like a Muun) from their video. Which can be interpreted as "too accurate and a spoiler", but of course also as "Not accurate" :)
This one isn't from the article linked (source), and of course is also at best barely remote circumstantial evidence:
But look back to 1983 and an interview in Prevue magazine with Return of the Jedi director Richard Marquand. In the interview he mentions having discussed the entire nine episode saga with George Lucas.
‘If you follow the direction, and project into the final trilogy, you realise that you’re going to meet the supreme intellect, and you think how is it possible to create a man who has such profound cunning that he can not only control Darth Vader, but the fate of Luke Skywalker? Control the destiny of the whole galaxy? You’ll be amazed!’
And we know from canon/EU that Muuns are highly intelligent overall (and Plagueis more so than average)
Minor circumstantial evidence arises out of TFA novelisation by A.D.Foster:
“Kylo Ren, I watched the Galactic Empire rise, and then fall. The gullible prattle on about the triumph of truth and justice, of individualism and free will. As if such things were solid and real instead of simple subjective judgements. The historians have it all wrong. It was neither poor strategy nor arrogance that brought down the Empire. You know too well what did.”
This means he's OLD. Older than Luke.