Shared Albums Tick Marked and Greyed Out, Not Syncing to Mac

Any other guesses? I repaired the Mac Photos library, disabled and re-enabled iCloud Photos, checked that the library is the system photo library and that it's synced with iCloud, that the shared albums are still there in iCloud.com/photos and in iPhone Photos. Shared albums is still check marked and greyed out, but all indications are that it's a false check mark, because I even tried creating a new shared album, and this option isn't available in the sharing menu

SOLVED!! Called Apple Support. When they tried to access my Mac via screen sharing I got a pop-up "Lockdown Mode prevented screen sharing". The support guy right away identified this as the cause why Shared Albums weren't syncing. With Lockdown Mode enabled, photos in your iCloud account can sync but Shared Albums (to which others can add photos, and which are separate from your own iCloud storage, as this article explains) may not sync. Turning this mode off (via Mac Settings > Privacy & Security > Lockdown Mode) immediately resolved the problem.

Yes, it is. That's a prerequisite for syncing with iCloud, it last synced minutes ago, and still no shared albums.