Virtual route for a static tag

[pixelijn]

@Peleke

// add the tag filter
if($tag = param('tag')) {
  $articles = $articles->filterBy('tags', $tag, ',');
}

Since there is no parameter set in this case, you just check for $tag, see the docs: Controllers | Kirby CMS

if($tag) {
  $articles = $articles->filterBy('tags', $tag, ',');
}