This question arised while reading through this question: Can Maul use Force Lightning?. There it is stated that Darth Sidious was loyal the Rule of Two in new canon.
From the novel Darth Plagueis, we know that
- Plagueis existed and was a Dark Lord of the Sith.
- Plagueis was the Master of Darth Sidious.
- Plagueis was alive during the events of The Phantom Menace.
- Plagueis was killed by Darth Sidious.
From these information we can of course conclude that Sidious disobeyed the Rule of Two, since Plagueis, Sidious and Maul co-existed during the events of The Phantom Menace.
But the novel Darth Plagueis is not canon anymore, so my question is:
What information is still valid in canon? And for the points not being valid anymore, what do we still know about them in canon?
If the author of the cited question is correct, than at least 3. must be false in canon. What about the other points?