In Super Smash Bros. Ultimate, various situations can cause some things to be disabled. This is done either due to technical reasons, creating unfair scenarios, or references. Examples include Arcade Bunny not appearing on Temple because it has a cave, Nikki not appearing on stages with dark backgrounds which will make her drawings hard to see, or Alucard not appearing on Wii Fit Studio because vampires don't have reflections in (silver) mirrors. Sometimes items will disable other items, such as Togepi and Palkia not showing up together because a Togepi using Smokescreen while the screen is flipped could result in more SDs than the two effects alone.
Assist Trophies
Assist Trophies that can or cannot appear on certain stages.
Poké Balls
Poké Ball Pokémon that can or cannot appear on certain stages.
Assist Trophy conflicts
Assist Trophies that can't appear when certain Assist Trophies are present.
Poké Ball conflicts
Togepi, Palkia, Victini, and Zoroark can not appear at the same time. All other Poké Ball Pokémon can appear with others.
Assist Trophy/Poké Ball conflicts
Assist Trophies that can't appear when certain Poké Balls are present and vice versa.
Other conflicts
Things that aren't Assist Trophies or Poké Balls that are restricted when certain Assist Trophies, Poké Balls, or other items appear.