I've been trying to reduce the storage that the Photos app uses, both on my Mac and in iCloud. I use Lightroom for my photo library, so I would like to delete all the photos that are not referenced by projects (calendars and photo books).
Unfortunately, Photos is unable to display any of my projects from before December 2018, so I guess those are lost causes. (Maybe in theory I could restore from a really old backup with older versions of macOS and Photos and prevent it from syncing with iCloud Photos?)
Focusing on the newer ones, my first thought was to create a smart album of photos that are not referenced by any projects/albums. Photos does have an Unreferenced condition, but I think that refers to files that have not been copied into the library.
This TidBITS-Talk thread recommends adding keywords to photos that are referenced, but that doesn't seem to be possible with projects and the current version of Photos. If I select photos in a project, there are no relevant menu commands enabled, and the Info window doesn't show anything about keywords.