When it comes to third parties and cuts:
Wouldn't be too concerned = Capcom, Sega, Bandai Namco
-These companies have been great when it comes to Smash ever since Brawl, or Smash 4 in Capcom's case, so I wouldn't be too concerned about them for Smash 6
Probably okay? = SNK
-SNK is a smaller company and seemed to be cool with working with Nintendo since they put a decent amount of their older games on the Switch, so I wouldn't be surprised to see them stick around for Smash 6. That said Ultimate is the first game we saw SNK stuff in, so we have no precedent with them when it comes to Smash, and we don't know what Smash 6 will be like, so I do wonder how prioritization will work with a smaller third party company like SNK.
Somewhat concerned = Square-Enix
Now, SE has been around since the end of Smash 4 and stuck around with Ultimate and even gave us much more than before, so I do think we will still see them with Smash 6. That said with how complicated and difficult getting content from them seemingly is, I would have some concern over what stuff actually carries over from Ultimate or if we get much on the side of new content.
Concerned = Konami, Microsoft
yeah, these companies are definitely on the iffier side. Konami we have to worry about because first of all, they are Konami, and secondly we have seen them dip from Smash bros after previously being a part of it between Brawl and Smash 4. ANd as for Microsoft, while they and Nintendo are buddy buddy now, who knows if that will still be the case years down the line?