It seems there are 140 packages on hackage using ListT (which surprised me, given its flaws), so I don't think we can quickly remove it from transformers or re-implement it under the same name

Don't worry. I've decided to keep it under `pipes` anyway. It seems like a more natural fit for it to be in `pipes` because it is otherwise very hard to build or consume `ListT`-done-right computations. So consider this proposal to be retracted.