"Smash fan-rules are more like guidelines".
This is what they should be, but it's just that some people take it to the extreme and think that its a definitive rule even when Sakurai doesn't explicitly say it is. I do think there is some merit to some fan rules. Like the fan rule of spirits and assist trophies deconfirming characters for the first fighters pass. That was a sound argument because it's based on the idea that those things were decided alongside the DLC so if, for example Rayman were to be playable for the first fighters pass, it would not make sense for Rayman's spirit to be right there in the base game. And people were right. It's the same thing with post-base game spirits. If they planned on adding a Resident Evil character, having that spirit event months ago would not make sense. Though that might not be the best example as there are a lot more characters who wasn't in that event that could be included in a future spirit board. The best example would probably be the Astral Chain or Ring Fit Trainer spirit events since those really limit any new spirits that a spirit board would bring.
That brings me to the second fighters pass and that's when I have a problem with the spirit and assist trophy rule. While it's hard to tell if the second fighters pass was possibly planned from the beginning, we can probably safely assume it wasn't. The thing is, I haven't heard a single convincing argument as to why spirits and assist trophies wouldn't be upgraded. When it at least comes to assist trophies those already give the Smash team a model and some animations to work with. People like to bring up that assist trophy upgrades have never happened in the same game but I would like to bring up that this is only the second Smash game to have DLC. For Smash 4, it was likely a bigger priority to bring back some veterans and introduce some more important 3rd party characters like Ryu and Cloud. While you could argue the same thing for Ultimate, the thing is that we already have every veteran and a bunch of new 3rd party franchises. We got the requested and iconic Castlevania series, the very influential Dragon Quest series, Persona which is a popular series with the fifth game being widely critically acclaimed and is considered one of the best rpgs of this generation, the heavily requested Banjo, and the series that directly influenced Smash, Fatal Fury.
While there are certainly a lot more important and requested third party franchises and I'm sure we are still getting some more, the thing is that third parties took over the first fighters pass by storm with only one new first party character. For the second fighters pass, we already got the confirmation of an ARMS character. Perhaps this is the time that Nintendo and Sakurai looked back at first parties and it just so happens that some of the most glaring and requested first party characters left (Waluigi, Isaac, Skull Kid, etc) are assist trophies. What I'm getting at is that this the biggest chance that assist trophies have ever had, especially since ARMS was announced and Spring Man is the mascot of the game. Same with base game spirits, plus the fact that Lucas and I think Mewtwo had trophies in Smash 4 before they became playable and that line of thinking could reasonably be applied to spirits too.
So the big picture here is that fan rules can be true, but they also need to adapt to new circumstances. Just because assist trophies and spirits were deconfirmations for the first fighters pass, that doesn't mean the same is true for the second fighters pass. Personally the only fan rules I agree with is post-base game spirits deconfirming and no 4th party characters. The former because of the same line of logic as my Rayman explanation from earlier on in this post and the latter because that is one rule that Sakurai himself keeps on hammering home.