You're right, at least on Android it seems we enable background sync by default and only allow users to change the default to block and re-allow specific sites (via Settings > Site Settings > Background Sync) - they can't block specific sites afaict.

It looks like this comes from only showing permissions that differ from default in that Secure dropdown on Android, which I suspect is done to reduce clutter. But given we allow users to explicitly re-allow certain sites with a textbox from Site Settings > BackgroundSync, I don't know why we don't use this kind of UI for explicitly blocking certain sites...

Cc'ing people who may know more.

I guess because the default is Allow and not Ask, we never create site specific content-settings for this feature.

It would probably be nice to show which sites are registered for background sync but I think the feature currently only allows to continue some request after a visit and no actual background features like execution in regular intervals so it doesn't seem too problematic. There is a section in go/chrome-pdd that explains it in more detail ("Background Sync API")

There are numerous inconsistencies like this on Android site settings/page info that need addressing.