I like how we were now casually discussing the etymology of one of the most tossed-out phrases used in the Smash Speculation. Yeah, disconfirm seems to be largely accepted word, since according to places like Merriam Webster dictionary, it describes the word as "To deny or refute the validity of..." So yeah, I probably should begin referring to dis-confirmations rather than de-confirmations. Just for extra authenticity.
But backtracking a little, in regards to characters I'd say have been explicitly disconfirmed, I'd probably say that Assist Trophy characters count until further notice, spirits that have been added post-launch for promotions like Astral Chain and Ring Fit Adventure on the grounds that they were probably too recent for playable status just yet, and characters who have been made into Mii Costumes in the various Mii Costume waves (Travis, Altaïr or other Assassins, Heihachi, etc). Rex is a very debatable case in the event he's chosen after considering Xenoblade 2 as post launch like they did later with Min-Min for ARMS, but it's a wait and see if you ask me