People have a bad habit of thinking that precedent is not evidence and then trying to use the fact that precedent can be overturned to bludgeon ideas built around precedence as "FAN RULE, DOESN'T MATTER" instead of taking a more objective step back to evaluate things and acknowledge that in the absence of more concrete evidence, precedent is one of the main things we can use to develop speculation beyond personal opinion.
And that's without even mentioning that in lots of instances, the reason for precedents being broken have generally speaking been more extraordinary than people like to admit. I mean, the classic example is that King K. Rool and Banjo & Kazooie broke the "rule of irrelevant characters not being able to get in." Yes, technically "irrelevant characters got in," but mind, they got in specifically with a new modifier of having enough fan demand to overcome that status. Overall, the trend of "irrelevant" fighters getting in hasn't really changed that much. If you're talking are a much more forgotten about character without that fan demand, there's still an overall precedent set that such characters are unlikely for Smash unless Sakurai adopts you as a pet project.
Obviously you have to address each and every "rule" on its own terms, but I feel some people use it to play into their deifying of Sakurai as this "unpredictable mad genius" of sorts when Sakurai has mostly just been a pretty consistent developer that has evolved over the years and has been willing to make unlikely realities, reality, through sheer determination and work ethic. Precedent is still and will always be an incredibly relevant source of evidence when making speculation and predictions. You're free to come to different conclusions based on it, but I do think sometimes people are quick to slap "Fan rule" on anything that pushes against their preferred choices or preferred understanding of Smash when as usual it's all just so much more complex than that.