Custom Move Sets, a feature introduced in Super Smash Bros. for Wii U and 3DS, will not return for Ultimate. According to GameSpot, a Nintendo representative stated this, and that equippable items will likewise not return.
Customs allowed players to mix and match fighter abilities. For example, Mario can shoot a gigantic, slow-moving fireball, or a weak little fireball that moves at an incredible speed. One of Pac-Man's customs is instead of his hydrant shooting out water, it shoots out fire instead. Instead of Kirby inhaling an opponent, he can exhale ice instead, freezing the opposing player. Peach could use her Toad counter to put opponents to sleep. There were numerous Customs to unlock. While fun, it was somewhat controversial. Customs were featured in the early competitive scene, but died out. You would be hard-pressed to find a major tournament in recent years that allowed for Customs.
Why was Customs not that popular? There are probably numerous reasons, but likely two core aspects. While fun, some Customs may make certain fighters overpowered, with the opposing player not having any powerful Customs to match. Second, is that it is very difficult to train competitively for tournaments that allow for customs. Customs were not enabled for online For Glory play, which is one way many players train. All of this equals to Customs not being a viable option for big competitive tournaments.
The idea of customizing fighters is not totally absent in Super Smash Bros. Ultimate. The Nintendo representative told GameSpot that Mii Fighter will retain the ability to customize. Spirits, a brand new mode in Ultimate, will allow fighter customization as you unlock various Spirit characters. (For example, equipping Vulpix will enable the fighter to have a Fire Flower.) While not exactly the same, Spirits looks to be an interesting successor to Customs. One positive aspect is the fan service; there are a lot of different Spirit characters. Dragonite, Ray MK II, Shantae, and so many more are featured. Whether or not the mode is popular after launch remains to be seen.
Super Smash Bros. Ultimate will be released on December 7th.