A new update: Now pinboard-API is used to show the number of unread bookmarks. This is much more efficient, but you have to login first.
So on enabling this feature, Safari will ask you to login to api.pinboard.in – enter username, password and click on “save to keychain”, that’s it. (If you don’t save to keychain, you have to log in each time Safari is started).
This is how it looks (that’s the German version):
Of course: Your personal data is only being transmitted to pinboard.in – the extension doesn’t have any access.