Meta is launching a new ad option called "Flexible Media", another step toward enabling variable ad delivery approaches based on systematic predictions of user response.

Okay, that was a bit of a jargonistic explainer. The more simple explanation is that Meta is looking to maximize ad performance by having more creative options that it can then display to different users.

On this front, Meta's already pushing advertisers to use its automated Advantage+ campaign tools, which do the work of effectively picking which of your ads to show different users. It also has several ad options that can customize and personalize ad display, with Flexible Media playing another part in this.
"Flexible Media" is showing up as an option to some advertisers on some Meta Ads campaigns.
Above its description for what "Flexible Media" does:

"We will deliver the media you choose for placement groups across placements when we think it will outperform."

To be clear, this is not to be confused with "Flexible Ads", which allows you to upload 10 images for an ad, and Meta's system can then choose and select which to serve to which user.

Flexible Media would seem another iteration of that; it would allow Meta's system to use different images for different placement groups. Therefore, essentially you're giving Meta's system permission to use different images for different ad types, as it sees fit.

As stated, it is another step towards Meta completely automating ad campaigns and displaying distinct ads to various users depending on how its systems determine the probable response. In summary, Meta is increasingly becoming more automatic in aspects of your ad process so that, effectively, you will not even have to come up with any element of your ad anymore. You will just simply put a URL, and Meta will do the rest.

Which probably feels a little like you're handing over too much control to the system, but since Meta's ad targeting process is built around its system AI, which it measures and tracks ad responses, it would be fully reasonable that it could help you reach a more receptive audience .

And you're probably limiting that by manually choosing your audience.

So even though this may sometimes feel wrong, and even though you're probably wasting your audience knowledge if you're not using that same knowledge to help target ads, you really probably should try Meta's automated ad options and see whether they do better. Another part of how this works appears to be Flexible Media, which seems to give Meta's systems more levers on which ads and displays to show to users.