I keep 60+ GB of photos on my iPhone, backed up via Google Photos. I don't use iCloud Photos and don't want my photos in iCloud. But iCloud Backup includes the entire on-device photo library whenever iCloud Photos is off, with no toggle to exclude it — Apple's own docs confirm backups include "everything on the device that doesn't already sync to iCloud."
Result: my iPhone backup can never complete on the free 5 GB tier unless I delete photos from my phone, even though I have local space and my photos are backed up elsewhere. The only options offered are buying more storage or deleting my photos.
Please add a per-device backup toggle to exclude Photo Library (and Messages) from iCloud Backup. Backing up "everything except photos" is a reasonable setup that currently isn't possible.