Super Smash Brothers Ultimate - NMH fans right now

True, but he didn't become playable until the next game that costume appeared in. I don't think Travis is deconfirmed forever but I do think it's unlikely he'll be in Fighters Pass 2 (maybe on a third one).

There's no official rule a character who is already a Mii Costume or an Assist cannot be a DLC Fighter later, people said the same thing about Spirits and Min Min proved that false.

It's possible that since Travis was revealed as a costume just recently, he can only become a DLC Fighter if Nintendo reveals a Season 3, but I think that characters who became Assists or Mii Costumes during launch may have a second chance now just like Min Min.