How to allow url processing only to fonts?

Not really sure what you mean with process fonts? Do you mean that you want to include fonts?

@bobbybouwmann This: https://laravel.com/docs/5.6/mix#url-processing

As far as I know you can simply load in the fonts using the url method as well

@font-face {
  font-family: 'Darkenstone';
  src: url('./Darkenstone.eot');
  src: url('./Darkenstone.eot?#iefix') format('embedded-opentype'),
       url('./Darkenstone.woff2') format('woff2'),
       url('./Darkenstone.woff') format('woff'),
       url('./Darkenstone.ttf') format('truetype'),
       url('./Darkenstone.svg#Darkenstone') format('svg');
  font-weight: normal;
  font-style: normal;
}

That should work out of the box as far as I know