This page is an archive of a community-wide discussion. This page is no longer live. Further comments or questions on this topic should be made in a new Knowledge Bank page rather than here so that this page is preserved as a historic record. Advanced Jedi Training Droid 6(Talk to my master) 06:01, October 28, 2016 (UTC)
If you look on plagueis' page then you'll read that he died prior to the battle of kashyyyk, basically before the events of Episode III: Revenge of the Sith for those of you who don't know what that battle was.
I'm assuming it's because that's the earliest mention of him that exists in the new canon, but we know from Episode I: The Phantom Menace and Star Wars: The Clone Wars TV Show that Darth Maul was referred to as "Darth" before his fight on naboo with Qui-gonn jinn and Obi-wan kenobi, which would imply plagueis' death taking place before the invasion of naboo, perhaps further.
First, please remember to sign all your comments with four tildes (~~~~) :). I think that the answer to your question is that while the Rule of Two and Darth Maul being referred to by the title "Darth" heavily suggests Darth Plagueis was already dead by then, we do not know that for certain and as such that would be an assumption. After all, in Legends it turns out that Sidious had violated the Rule of Two by training Darth Maul while Darth Plagueis was still alive and that he was still alive for parts of The Phantom Menace, which demonstrates how the simultaneous status of Darth Plagueis, Darth Sidious, and Darth Maul as Sith, a violation of the Rule of Two, is possible. Whoever went with "before the Battle of Kashyyyk" may have been mindful of this possibly being true in Canon as well. We only know without any doubt (but then... it is only by Sidious's account), that he was dead by the Revenge of the Sith. Also for future reference, I think that questions specific to a single article, especially corrections, should be asked in that article's discussion page. The Knowledge Bank's purpose is to focus on questions regarding lore, but then again you might've been asking from a lore-standpoint not a corrections-standpoint. Sol PacificusTelepathy 00:19, September 28, 2016 (UTC)