one sits down with Sakurai and does and interview, they never ask him "do assist trophies deconfirm characters?". Nobody has to ask that if they don't want, but that's a burning question in the Smash community that has pretty much gone unanswered for over a decade. Odd that nobody has brought that up to him.
Because it has a very obvious answer that only the smash community seems to not get?
When sakurai develop a smash game, the roster is one of teh first things to get decided. regarding character concepts, moves and whatnot. he probably choose a mixture of character that offer popularity (Ridley, Krool, Wolf), relevancy (Isabelle, 7th gen pokemon, Joker), and uniqueness (Piranna Plant, Inklings, Ice Climbers.) obviously this chaacter may be fine in more than one point (ridley is still relevant, Isabelle and inklings are also popular, etc..) Once they decided that, which characters are the ones that will burn most of the development time they look at which popular characters could work as little extras as echo characters (Dark Samus, Ken, Chrom) after that. he sees the list of characters that didn't made the cut and choose again, popular ones (knuckles, bomberman, zero, alucard), some relevant ones (springman) and a couple of unique ones (The moon) .
now.. some time during that development they start discussing DLC... obviously they aren't going to say anything that is not set on stone. but nintendo, not sakurai, have an idea of which characters want to pursue as DLC, they still probably talk it with sakurai (not goku!) and probaly at this point (that the game havent been revealed) there is a vague glimpse of what nintendo idea may be with DLC (like, promote big third party names).. at this point teh people on teh development team knows that Waluigi, Lyn or whatever didn't made the cut, even if any details regarding DLC still may be super hazy and in the "discussion" and "may not even happen" phase.
So yeah, they develop those assist trophies and show them.
thats why assist trophies aren't going to be on the game during this DLC cycle. if they wanted to put them on the game they wouldnt be assist.
THAT being said, if Sakurai, or nintendo eventually wants to make a second DLC cycle, and they decide, "we want to put a super popular character with the community as one of the fighters" they aren't going to say... "uhmmm... this waluigi guy is loved by many, but sadly he is already an assist trophie, so let's go with pink gold peach instead, since she is not an assist".
If they need DLC characters within an specific criteria and somehow an already assist trophy fits that criteria perfectly, they will make it a playable character, thats a given. but thats is an hypotethical assumption.. at this point they probbaly still evaluating ultimate's fighter pass performance, and there is always the chance that sakurai simply don't want to work another year on it.