In fact recently it was spotted on IMDB that Goku was listed as a character with voice actor Sean Schemmel attached to voice the English version of the character. Now Schemmel has done a lot of voice acting for Goku in the past, so to see his name listed made sense, although the bad news is that it turns out that this might not be true.
I have not recorded nor have I been approached by anyone from Nintendo or otherwise to record for this game. I am equally perplexed.
— Sean Schemmel (@SeanSchemmel) August 9, 2018
Responding on Twitter regarding his name being listed on IMDB, Schemmel confirmed that he has not been approached by Nintendo or anyone else to voice Goku in the game. Apart from Schemmel’s resume, it is easy to see why this might be believable. This is because Bandai Namco is said to helping Nintendo with development of the game, which could mean that maybe as part of the deal, Goku could be added to Super Smash Bros. Ultimate.
That being said, it is more likely that the listing was made in error (or someone trying to spread rumors), but an alternate possibility would be that maybe Nintendo decided to go with someone else. Either way until Nintendo or Bandai Namco announces something, take it with a grain of salt.