Adsense Related Search Experiments Temporarily Paused Till July 2021https://support.google.com
Beginning May 10, 2021, you will not be able to create new custom search style experiments for your Related search styles in your AdSense account. Your existing Related search experiments that are currently in progress will also stop on May 10, 2021. Please note that this change does not prevent you from creating experiments for other elements of your styles (e.g., search ads, shopping ads).
Our engineering teams are working hard to bring this functionality back by early July 2021.