Relude reexport the ExceptT monad transformer (together with runExceptT). But that's all. The typeclasses MonadReader, MonadState, MonadIO and MonadTrans are all included in Relude, but not MonadError.

How are we supposed to use ExceptT when its corresponding interface is not included?

PS: The typeclass MonadFail is something else that is not related to ExceptT.