Threads in all sections are removed from view automatically when they get past 30 days old. They are still on the site and searches reveal them, but there is a limit to what is displayed, and the cut off point is nothing 30 days old.
In this instance, there has only been 1 site suggestion in the past month, so only that one is visible. It just means this section, which is a sub section of another poorly used section, is not very popular.
Mods or Eds do not remove any threads because they do not like suggestions.
As others have said, there are suggestions, there just aren't any under the default 'most recent' tab because none of them are recent, but you can still see them under the 'answered' and 'unanswered' tabs.