While glancing through the Attack on Titan Guidebookrecently, I noticed something pretty surprising in the "White Paper" section (a big in-universe infodump about the military) that I'd completely forgotten about.
In the section of the White Papers detailing the equipment and technology of the Survey Corps, the Papers describe some technologies that are mentioned only in the Before the Fall spinoffs: iceburst stone and iron bamboo. While not specifically mentioning events and peoples from Before the Fall, the details do line up with what's described in Before the Fall.
Since the Attack on Titan Guidebook was written and illustrated by Hajime Isayama himself, the White Papers included (as far as I know), it's a completely canon source. Isayama must have drawn these two elements from Before the Fall and approved of them, including them alongside Vertical Maneuvering Equipment and ultrahard steel, his own original concepts.
Basically what I'm getting at is, how does this detail affect the canonicity of Before the Fall on the Attack on Titan Wiki? Would this be enough to consider Before the Fall canon, whether it's the novels or the manga?
It could also be the opposite case. Whenever a spin-off is in production, the mangaka is called to give advices to the writers about the setting and elements of the story (not only in AoT's case but in pretty much any manga). Maybe he told the Before the Fall team about the Iron Bamboo and the Iceburst Stone and they applied them to their story before Isayama wrote the guidebook. So I don't think this has any effect on the current non-canonicity of Before the Fall, we need to find an unquestionable proof; preferably a character or an event from the spin-off affecting the main storyline somehow. Items are too diffuse.
This is simular to when Eren called Jean "Jean-Boy" in chapter 72, it doesn't mean that the OVA, Sudden Visitors, is cannon. Besides, Before the Fall can't be cannon because it has night-walkers, something that isn't introduced until chapter 37ish.
These are all valid points being made and I agree with them, but when we saw images in Chapter 75 and 80 that alluded to the No Regrets spinoffs, we declared that to be canon. Where is the line drawn between confirmation and allusion?
There is no clear line, these situations need to be approached case by case.
In Ms. Kirstein's case, she first appeared in the extra pages of Vol. 6, and she referred to Jean as "Jean-Boy"; or at least with the word that in fan translations was spelled as such (in the official Kodansha USA version she just says "Jean"). She is a canon character that was later depicted in a non-canon source.
The same situation applies for the Iron Bamboo and Iceburst Stone. We have no way to know if any of those items was a creation by the Before the Fall staff or Isayama's creations that they adapted into their work; and even if it was the first case, the fact that Isayama took two Before the Fall items to the canon storyline doesn't necessarily mean that he took the whole story with them. Those two elements can perfectly exist without anything else.
No Regrets is a totally different situation. We got the first hints that the story was canon in the afterword that Hikaru Suruga included in No Regrets Vol. 2, where she thanks Isayama for "letting her write Levi's spin-off"; suggesting that Isayama had accepted it as Levi's official background. Then in chapter 76 we got a direct flashback of a No Regrets scene, and then in chapter 80 Furlan and Isabel crossed over on the main storyline. Both the flashback and the characters' presence would have absolutely no meaning if the events of No Regrets never happened. That's why this manga is now accepted as canon.