Looking through the latest ImprovementDefs.xml, there are quite a few improvements that simply can't show up. I don't think the code editor understood how some commands, such as IsPlayerWonder, IsColonyUnique, UpgradesFrom, Unavailable or Preclusions, all function together. They're fairly obvious mistakes.
They're mostly Tourism, Cultural and Financial upgrade buildings. Some of them were working fine before, but stopped functioning with 3.0 and 3.01 Preview.
Hopefully they get addressed with next patch.