bootstrap/scss/utilities/_display.scss

// stylelint-disable declaration-no-important

@each $breakpoint in map-keys($grid-breakpoints) {
  @include media-breakpoint-up($breakpoint) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

    @each $value in $displays {
      .d#{$infix}-#{$value} { display: $value !important; }
    }
  }
}

@media print {
  @each $value in $displays {
    .d-print-#{$value} { display: $value !important; }
  }
}