Google has just introduced the Locked Folder feature for Google Photos for non-Google Pixel devices.
Locked Folder is a secure on-device folder where you can store private videos and photos. These won’t be uploaded to Google Photos as well as can’t be shared, screenshot or screen recorded unless it’s removed from the Locked Folder.
The new feature can be found by going to the Library tab in the Photos app, then going to Utilities, and scrolling down to Locked Folder.
The feature was introduced last June for Google Pixel devices.