Filter a collection by taxonomy tag and exclude entries with a specific taxonomy.

Statamic: Exclude collection entries by taxonomy tag custom filter.php

<?php

namespace Statamic\Addons\StartTax;

use Statamic\Extend\Filter;

class StartTaxFilter extends Filter
{
    /**
     * This excludes entries that have a specific tag set. In this case 'Startseite'. 
     *
     * @return \Illuminate\Support\Collection
     */
    public function filter($collection)
    {
        return $collection->filter(function ($entry)
        {

            $entry_tag = $entry->get('tags'); // looks for tags array.

            if(empty($entry_tag)) // if no tags are set, no 'Startseite' is set as well so output.
             {
                    return $entry;
             }

             // if there are tags, we need to know if 'Startseite' is active.
            if(!empty($entry_tag)) {
                $check_for_startseite = in_array('startseite', $entry_tag); // check if tag 'Startseite' is set.
                // If no tag 'Startseite' is set output the entry.
                if(!$check_for_startseite) {
                    return $entry;
                }
            }
        });
    }
}