Skull Kid is still missing from the game entirely so far. I seriously doubt Skull Kid, who was just added in Smash U, would be removed entirely when other old outdated Zelda characters Assists still remain like Midna, Giraghim, etc. And we have the Moon without Skull Kid! How many times have people said it would have made far more sense for Skull Kid to summon the Moon and teleport away? Sakurai knew people would talk about that.
They even made the Majora's Mask a Mii outfit last time. It just doesn't make sense for Skull Kid to be so mysteriously absent. And I just cannot see him as a boss so I don't know what's up.
I mean everyone noticed Shadow's Assist trophy was missing and lo and behold he's on the leak and people expect him as an echo. I just cannot comprehend Skull Kid's complete removal from Smash if he wasn't intended to be playable.
Honestly, this right here is what has me feel like Skull Kid is playable, as base or DLC (whether in the banner or not since we don't know for certain if it's even real, let alone if Skull Kid is in it). Because it seems almost like he's cut from the game entirely and doesn't make much sense why. Some people might say "Oh, well Isaac's Assist Trophy disappeared for one game and he wasn't playable then, that means nothing!" but it's not quite the same. For Skull Kid...
-he had an Assist Trophy last game
-he had a Mii costume last game, like several characters such as Isabelle, Dark Samus, and Geno (IIRC). The first two characters are playable and one might be
-Skull Kid was initially not in Hyrule Warriors as DLC, but fan demand had them put him in. Skull Kid has popularity I know that this one might not be as good a point because he debuted in Legends in 2016 after the roster was finalized so that could be a point for DLC, but I digress
-Majora's Mask was rereleased for the 3DS in 2015 before the final roster was decided, bringing Skull Kid back to the public eye again
He has a fanbase. He has a presence in Smash. Several Mii costumes have been promoted to playable.
In Smash Ultimate, what we definitely know so far:
-his Assist Trophy hasn't been seen, appearing to be replaced by the Moon and that new mushroom AT. They could have just had Skull Kid call down the moon but he's not there. Likewise Shadow's AT is missing and replaced with Knuckles, and if the Grinch leak is at all accurate, he's playable
-Young Link's Final Smash, which could be Fierce Deity, has not been seen and would be a perfect counterpart to Skull Kid
-"He could be a boss", you say?
Then why haven't they shown his Assist Trophy yet? It was established in an earlier Direct that characters can be both Assist Trophies and boss fights. They could have shown his Assist Trophy earlier if he was still one. But much like Shadow, he's nowhere to be found.
And if we take into account Chad Ganandorf's leak with any credibility at all, then Skull Kid's not playable, he's not a boss, and he wasn't cut from the game. So if that leak's taken with any sort of legitimacy he was just... left out entirely? Given the last game and what we know about this one, it doesn't make any sense.
Could he be first in line for DLC? Absolutely. It's just all really really strange...