Just last month, Smash Bros. Ultimate got an update that allows anyone playing on Final Destination, Battlefield, Big Battlefield, and Small Battlefield to choose from the over 1,000 song library that the game has to offer. While implementing this feature was a huge amount of work in and of itself, Masahiro Sakurai had get a lot of paperwork signed off on as well.
According to Sakurai, permission from all the third parties involved with those songs had to be given. This wasn't something that was covered already in the Smash Bros. Ultimate agreements, so Sakurai and his team had to work the music selection out with individual companies.
"Getting permission from all the rights holders for something like this is a very big undertaking, but somehow we managed to make it a reality."
No one doubts how dedicated Sakurai is to the Smash Bros. series, but this is just one more example of how far he's willing to go.
Thanks to Dondom95 for the heads up!