When it comes to "Dragon Ball Z", the fans can be sticklers for what is "canon" and what isn't. Certain sequences, even an entire mini-arc, can and have been dismissed as non-canon. The test is largely based on whether or not it happened in the manga. If it happened in the manga, it's canon; if it didn't, it's filler.
The big problem with this is the fact that this isn't how canon works. The anime and manga are two independent things. Liberties are taken with adaptations all the time. Changes are made for various reasons, but the canon of the adaptation remains its own thing. This holds true for the anime just as much as it would an adaptation of any other movie, comic, or book. These changes may not be canon for the manga, but why wouldn't it be canon for the anime timeline? For that matter, why is this franchise the only one burdened with this condition? You don't see other franchises endure this particular method of scrutiny to determine canon validity.
Furthermore, this really doesn't make any sense. What about instances where story arcs play out differently? You see this in "Super", where both the Goku Black arc and the Tournament of Power arcs play out in wildly different fashions. The events as depicted in one really have no effect on the other and it would be weird to ask fans to view or read it that way.
By this logic, you could dismiss just about any adaptation. A good chunk of the MCU or "Game of Thrones" would be thrown out because that's not how it played out in the source material. Admittedly, from what I've heard, a lot of people have "non-canoned" the last season of the latter, but that's more a criticism of quality than anything else.
This is another angle that can fuel plot point erasure. This one is a bit more understandable, if there's a thing that you really don't like, you choose to ignore it. Going back to the MCU, this happened with "The Inhumans". It was very poorly received to the point where neither the fans, or even the creators, really acknowledge it all that much.
Even here, though, the events will usually still have an impact on the story. You may or may not like it, but it's still there. You can say that it isn't part of your personal "head-canon", but its canon status still remains intact. For example, a lot of people don't like how the Goku Black arc ended and choose to go with Trunks being the one to score the final win against the omnicidal adversary, but the Zeno/deus ex machina ending is still the ending. If everybody picked and chose what counted and what didn't in terms of storytelling or story adaptation, the whole process would be rendered moot. The whole concept of "canon" would be thrown out the window with this approach.
It would be one thing if the anime and manga were more synchronized. "Star Wars" for example has a single canon that spreads across multiple mediums. Well, two canons now, what with the erasure of the original "expanded universe", but you see my point. If the anime and manga were meant to be part of one cohesive universe, then arguing canon or non-canon makes more sense. That's not really the case, though.
Between the multi-verse and alternate timelines, you could easily have your cake and eat it too by saying that it happened, just in an alternate timeline. That seems to be the case with "GT". Sure, it wasn't very well received, but you could still easily view it as an alternate timeline where Beerus didn't wake up.
So, in summation, unless directly stated, canon is self-contained to its respective medium. As such, we can conclude that the title of this post holds true, despite the ardent assertion to the contrary.
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