! was the kind for effect indexing. In older versions of PS Effect was a kind, and Eff was the effect type indexed by things of kind Effect. Aff was similarly indexed in this way, but that was dropped during the transition to purescript-effect. It looks like the particular file you are linking to in the gist is just copy-paste duplication though, as it has the old file mixed in with the new, making invalid syntax.
Thanks, natefaubion, I appreciate the context and indeed it is currently invalid. Are you aware of any migration resources (documentation, posts, or code tools?). I can’t yet recognize old vs new syntax by eye…