The first few generations that were sold with the promise of unlimited photo backups still get that deal - if you find an old Pixel / Pixel XL and use it to upload your photos, they will not count towards your storage. A few more models then get unlimited uploads in “high” quality, and everything since I think Pixel 5 is completely out of luck.
I’m not a native English speaker so 🤷♂️, but the term disk quota is commonly used to refer to this kind of limit.