They've never explicitly said which characters were intended to be promotional, and they probably never will.
Regardless, even though we can't prove anything, there are several characters who have had very convenient and suspicious timing. Bayonetta having files in the game as early as a week or two after the ballot began is fishy, especially when you consider how Nintendo in recent years had shown interest in funding her games so they'd be exclusives.
Then there's Byleth, who was said to have been chosen by Nintendo. If Sakurai feels the need to lower people's expectations and calm the atmosphere right before the reveal in the Byleth presentation, then either Nintendo forced him to add a character he didn't think would be well-received, or he lied and chose the character himself despite knowing the common complaints of too many FE characters. I'll take his word and assume the former, and if that's the case, what reason would Nintendo have other than promotion to push for Byleth when Sakurai knew it would be very controversial?
Again, we'll probably never have solid evidence, but I think most people can agree that Nintendo steps in from time to time to cash in on Smash's ability to advertise.
According to an article he penned shortly after Byleth's reveal, it was because he thinks having a very new character in a newcomer group made of mostly older ones makes the group as a whole more interesting!
I never thought that explanation made much sense considering how the first and second characters in the pass were also very new and hardly qualified as old. Joker is from a newer game no matter how you slice it, and Hero was promoted as a DQXI rep despite having three alts from older games. You could say that he was talking about the age of the franchise and not the characters, but even then it doesn't make sense because FE is old as well.