Ability to add custom fields no longer working

The docs for this plugin say there is a box in the search appearance tab where you can add ‘Custom fields to include in page analysis’ but that box is no longer included in the plugin. I’ve installed it on three sites now, including one with no other plugins installed other than Yoast, so it doesn’t look like a compatibility issue. No answers on the plugin support page and Yoast will only look into issues for premium subscribers unfortunately. Very frustrating.