I recently updated Google Photos (Build version 1.23.1.126715090) on my Android phone and noticed that my photos are no longer set to automatically sync to the cloud. I went into the app's settings and I cannot find the option that formerly existed: "Backup while on WiFi only."
Is this feature hidden somewhere or did Google (perplexingly) remove it? I really counted on this to save on data usage, especially when uploading video.
OK, so I did some testing and have determined that although the wording on the Photos "Settings" screens has changed, it is still possible to back up your photos solely when the phone is connected to WiFi.
On the Settings-->Back up & sync screen, enabling "Back up & sync" means "WiFi ONLY" by default. Lower on the Settings screen, in the "Cellular data back up" section, enabling "Photos" means that you are allowing photos to back up when on your cellular network. Leaving this switch off will prevent cellular backup of photos, but by virtue of having "Back up & sync" enabled at the top of the screen, your photos will back up via WiFi.
See the screen shot below. I have drawn in red to emphasize the toggle switches mentioned in my answer.
Q & A