The Pixel 4 has some impressive new camera tricks up its sleeve, such as live HDR+ previews and Dual Exposure Controls, that should help you shoot even more stunning images than you could with its predecessors. Even though these features seem like they must be purely software-related, apparently they aren't. That's why they won't come to the older Pixels, as Google has just stated on Twitter.
The company confirmed as much in response to a question about whether Dual Exposure was Pixel 4 exclusive, while also stating that Live HDR+ wouldn't come to previous Pixels, either. Both features "require low-level capabilities in the hardware," so there's no hope that we'll ever see them on the older devices officially.
Hi Mahfooz, Dual Exposure Controls and Live HDR+ require low-level capabilities in the hardware that are only available on Pixel 4, so they will not be available on older Pixel devices.
— Made by Google (@madebygoogle) October 21, 2019
Pixel 3 and 3a owners are still going to see some capabilities first introduced with the next-gen phone. Astrophotography is already live in Google Camera 7.1, and Top Shot for video is slated to come to the older models, too.