Off topic but I never knew the term for this. The cbcnews.ca site got rebuilt in react recently and the result has this one bug where when loading, some headlines flash an entirely different but also valid headline for the same story for a split second. When it comes to news, a FOUT like that makes me rather paranoid about maybe I'm getting a tailored headline for my demographic.
It's been really common for years to split test headlines[1] for content. Even most email marketing platforms have a variation of it, where they stagger out emails, sending out a few variations with different subject lines to a subset of users first, identifying the one with highest engagement, then releasing the email to the remainder/bulk of your email list using that subject.
I wouldn't put it past the major publishers to have moved beyond A/B testing for an overall headline winner to more segmented optimization based on tying A/B performance to first and third party data they have available on you, and further optimizing their headlines to reflect whatever performed well for traffic like you, rather than just performed best overall for all traffic.
