After an eight-month hiatus in May 2021, Google has resumed the AdSense-related Custom search style tests. “As of January 6, 2022,” Google added, “we’ve re-enabled your ability to create Related search Custom search style experiments in your AdSense account.”

Google AdSense Resumes

The hiatus was originally planned to last a few months, from May to July 2021. But I suppose it went on for a little longer. Google noted at the time “You won’t be able to create new trials for your Related search types in your AdSense account after May 10, 2021. On May 10, 2021, your current Related search trials will also come to an end. Please note that this change does not preclude you from conducting trials for other components of your advertising (for example, search ads and shopping ads).”

You may get to this by going to Optimization, then Experiments, then creating a Custom search style; but, you may run into an issue soon.