The thing that’s always jiggling in my brain about the nature of comics canon is that, theoretically, yes, the possibilities of what’s canon are limitless; canon often reboots and contradicts itself. But practically speaking, there is still a finite number of things that have ever been established in canon.
Like there is a point at which you leave “canon self contradicts/is ambiguous about this so you could choose anything and you’d still be working within at least one version of canon” and enter “this is fanon and diverges from what has been established in any canon so far.”
And for the record, there’s nothing inherently bad about something being fanon instead of canon. Things like fanon are a natural part of interacting with stories! However, I do think sometimes people see the variance in canon and conclude that anything is canon, when that’s not quite accurate & can leave some misconceptions about parts of canon that are actually pretty clear and consistent.