virtual/ruby-threads no longer serves a purpose since all currently supported ruby slots have mandatory threading support. Ruby 1.8 was the last version to make this optional, and 1.8-based versions are no longer available. Currently dev-ruby/ffi is the only package that still depends on it. Stable bug 537072 has been filed to stable a newer version that no longer depends on virtual/ruby-threads. Once that is stable the virtual can be removed.
This package is now masked for removal.