node_modules/flask/sass/font/_font-stack.function.scss

// Font Stack Function
//
// Returns a full font stack based on the given fontface and its variant.

@function font-stack($family, $variant: 'regular') {
  @warn '`font-stack` will be deprecated in verion 1.0.0. Please use `font-family` instead, which uses the same signature.';
  @return font-family($family, $variant);
}