Perfect!
Now I extended my pluck like your hint on Pluck datefields - #7 by TakioTk like this:
$y_filters = $unfiltered->flip()->pluck('date', null, true);
$y_filters = array_unique(array_map(function($item) {
return date('Y', strtotime($item));
}, $y_filters));
and the filter menu like this:
<?php foreach($y_filters as $year): ?>
<a href="<?= $page->url() ?>?yearfilter=<?= $year ?>"><?= $year ?></a>
<?php endforeach ?>
Thank you very much!