Update: Button $btn-disabled-opacity was added by BS4 at twbs/bootstrap#24680, don't need the extra button styles anymore

Author: pat270

src/scss/components/_buttons.scss

@@ -34,13 +34,6 @@
 	}
 }
 
-.btn[disabled],
-.btn.disabled,
-fieldset[disabled] .btn {
-	// Will need to be revisited if https://github.com/twbs/bootstrap/pull/24680 is merged
-	opacity: $btn-disabled-opacity;
-}
-
 // Button Link
 
 .btn-link {

src/scss/variables/_buttons.scss

@@ -3,8 +3,6 @@ $btn-font-size-mobile: null !default;
 $btn-padding-x-mobile: null !default;
 $btn-padding-y-mobile: null !default;
 
-$btn-disabled-opacity: null !default;
-
 $btn-font-size-lg: null !default;
 $btn-font-size-lg-mobile: null !default;
 $btn-padding-x-lg-mobile: null !default;