By accident I found out that on this site the privilge reduced ads at 200 reputation does not exist according to the help page. Is there a reason for this?
This answer seems to suggest there are no ads on Chemistry (except for the sidebar) and the privilege is therefore hidden:
The privilege exists everywhere. But on most sites, it's hidden and notifications for earning it are suppressed. Doesn't make much sense to tell folks about a privilege that doesn't actually do anything.
Here is another answer which more or less claims the configuration is up-to-date.