twbs--bootstrap/scss/mixins/_transition.scss

@mixin transition($transition...) {
  @if $enable-transitions {
    @if length($transition) == 0 {
      transition: $transition-base;
    } @else {
      transition: $transition;
    }
  }

  @media screen and (prefers-reduced-motion: reduce) {
    transition: none;
  }
}