Disney does own almost everything in Kingdom Hearts but Verum Rex is purely Square Enix owned since its based off Versus 13
It depends on the business discussions that would've been held behind closed doors. Although, since Verum Rex is
technically a Square Enix title in Toy Box... It's possible that Verum Rex is an IP that's already Square Enix-owned and just hadn't been shown until KH3 came out. (HMK's deductions, mixed with my own ideas.) If that isn't the case though and there wasn't any discussion with Disney beforehand, then Yozora is also a Disney-owned character, as well as anyone else that'd appear a Verum Rex-exclusive character.
And that's why I found the whole "Verum Rex is going to get rid of Disney worlds!" idea silly. Just because KH3's Secret Ending was on the darker side, it doesn't mean Disney's getting the axe (they legally can't anyway lol).
Another Side, Another Story is a pretty dark-looking secret ending... And the game it was alluding to had Sora singing dorky Atlantica songs, Goofy getting smoked by a boulder, GET UP ON THE HYDRA'S BACK, literally Demyx's entire existence, and a two-hour prologue consisting of teenagers spending their last week of summer vacation doing kids stuff. This game's ending had an even
darker secret ending... And
that game had Ice Cream Beat, Captain Justice/Dark, Mickey getting yeeted throughout the galaxy because of a Star Shard, "Information: Put Peter Pan in his place!", all of Disney Town and Go Kart Racing.
Kingdom Hearts games are dark, but they're not Drakengard/Nier/Persona 3-levels of dark lol. At least, not in the mature aspects of what those titles have. Verum Rex looks dark but might very well turn into your typical KH game where the overall theme is dark, but you have goofy and lighthearted moments all over. Even DDD had it, like Goofy yeeting everyone out of the tower.