During the Made by Google event, the presenters of various new Pixel 3 and 3 XL features promised that at least some of them would also be ported to their predecessors through software updates. Now, however, the company has confirmed that new camera gimmicks such as Top Shot will not be among them.
https://www.notebookcheck.net/Whole-load-of-new-Pixel-camera-functions-not-coming-to-older-Google-phones.338608.0.html