GHC.Event.Unique vs Data.Unique

Is there a reason GHC.Event.Unique exists, since we also have Data.Unique? Or is this just a historical artifact? It looks like they used to have the same implementation, but have now diverged, with Data.Unique being the more recent one. GHC.Event.Unique still uses STM internally, and is as far as I can see the only part of the base library to do so. Which wouldn't really matter, except that now basic IO stuff like threadDelay and even putStr (sometimes, especially in ghci) cannot be used inside unsafeIOToSTM. Which is somewhat annoying…