A developer has created a Google Camera mod that enables portrait mode on devices such as the original Pixel series, Nexus 5X, and the Nexus 6P. This mod enables the Pixel 2 exclusive portrait feature on older generation Google devices.
https://www.notebookcheck.net/Camera-NX-brings-Portrait-mode-to-the-original-Pixels-Nexus-5X-and-Nexus-6P.275762.0.html