Agreed, especially on the matter of what is "eligible."
We ought be wary of labeling newer elements as deconfirmation in the same vein as Assist Trophies. We can assume with a reasonable degree of certainty that ATs will not also be Playable due to such an event never occurring throughout multiple sequels, which established a clear and consistent pattern. Due to this observation -though nothing says an AT can't be a fighter, and the possibility of it happening does technically exist- it's "safe" to consider such an occurrence statistically improbable, making ATs the closest we have to "Hard" deconfirmation.
The same can't be said of Mii Costumes. There are already several examples of playable fighters with costumes, such as Fox, Captain Falcon, and Link. Of course, one can argue that no character has had a Mii Costume before being added as DLC, but that's much shakier ground to stand on. While the claim is not untrue, there have currently only been 7 DLC fighters added in Smash post-Mii Costumes, so it has an incredibly small sample size to draw a conclusion from. We also do not yet know if the Inkling, Isabelle, and King K. Rool costumes have been removed, which adds further ambiguity to the idea. So then, the most compelling argument we have for "Mii Costumes won't be added for future DLC characters" is the idea that it would be at least a little strange to spend resources to develop a Mii Costume entirely as a temporary holdover. That argument still holds water, of course, but I would consider it more of a "Soft" deconfirmation, as it diminishes odds, but does not seem sensible to assume it entirely removes characters from the running.
Spirits, on the other hand, are a complete unknown that we simply do not know enough about to make calls any which way on. Assist Trophies, Mii Fighter Costumes, Pokeball Pokemon, Stage Hazards, etc have Spirits. Even if we assume a complete absence of playable fighter Spirits, until we actually get DLC in a post-Spirits world, we have no frame of reference by which to ascertain their capacity as a deconfirmation notice.
P.S.: I appear to have missed a long conversation about KOS-MOS two days ago; woe is me.