I don't think the term "fan rule" itself is necessarily wrong, and there are times when "fans" act like a pattern is a "rule", despite the fact that Sakurai nor NIntendo themselves have confirmed it to be a rule. Now I'm not aiming this at people that are perhaps skepticle of certain things happening like, say, AT/Mii costume promotions, I'm not going to judge people for being skeptical, but I'm moreso talking about people that act super confident or downright arrogant over these things; people that act like something is 100% the case despite, again, the fact that no one at Nintendo or Sora ltd saying as such. For me there's a huge difference between "Eh, I'm just not confident this will happen" and "OMG, ATs are not happening and are totally out, just get over it already!" or something like that.
I'll also add that I have definitely seen annoying behavior on both sides here. As I said, it can be a pain when people try and shut down discussion on ATs being promoted, but at the same time I certainly found the "spirits don't disconfirm!" side to be tiresome at times, or how they'd completely warp or misinterpret the "spirits disconfirm" side. The thing is, many people on the "spirits disconfirm" side likely were thinking that spirits disconfirmed only for the first fighter's pass. And that DLC past the first FP could perhaps see spirits and maybe evn mii costumes and ATs back on the table. This and of course there's the fact that teh first few DLC characters we got were, in fact, characters with no content or even spirits to speak of. Despite all of this though, I still had to see a bunch of people spout "B-but just PNGs!"
The overall problem here isn't the term fan rules, or if people follow them or not, the problem here for how much some people try to shut down reasonable discussion or speculation.