Some private courses are listed as extinct even though they still exist. You can search for them by checking "show extinct courses". You can also still a mark an extinct course as played. I've marked at least half a dozen "extinct" courses as played over the years. Most courses are on here somewhere unless they are relatively new. Furthermore, that is one way "around" being able to mark a course without it being viewable/searchable for the masses. It can be added and then immediately extincted by Tim. I wouldn't really advocate adding a private course without the owner's permission though.
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