"With Pixel 2 you get free, unlimited original-quality storage for photos and videos uploaded through the end of 2020, and free, unlimited high-quality storage for photos uploaded afterwards. After 2020, there is no change to the photos you took before 2020 and you will always have access to those in original quality.

This is new for this year. We know that people tend to change their phone every 2-3 years so for Pixel 2, we are offering storage in line with that."

Google Photos will limit Pixel 2 users' uploads after 2020 https://www.engadget.com/amp/2017/10/05/google-photos-pixel-2-limit/