It looks like there is a section called "Smart features and personalization" in the GMail settings (not the general Google account settings; specifically the GMail settings) under the General tab.

It indicates that "Gmail, Chat, and Meet may use my email, chat, and video content to personalize my experience and provide smart features. If I opt out, such features will be turned off."

It's strange that this major privacy setting is buried specifically in the GMail section and not mentioned in the main Google account Privacy Checkup. I never knew it existed.