Google Adsense has announced that it has renabled its related search experiments. If you log in to your AdSense account, you will now be able to customize and create related search experiments.
Google Custom Search Experiments
Google Adsense paused the custom search experiment on 10th May 2021 as its engineers worked to bring the functionality back. The feature is mostly utilized by websites with high informational search traffic like wikihow.com.
Only one Custom search style experiment can be created at a time.
To create custom search style, login to your AdSense account>Optimization>Experiments>Custom Search Style>Select ad style>
NOTE: This feature is not available in every AdSense account. You need to apply for AdSense Custom Search Ads to get the feature