Do folks think additional verbiage is required here?

Are the terms in that verbiage explained elsewhere in the spec?

It's used verbatim in Annex B but not defined up front AFAICT. I plan to discuss it at the next editor call.

At the editor call, the decision was that we would stick with the phrasing "normative but optional". On a case-by-case basis, if a particular normative optional feature is additionally constrained by upstream specifications, we would link directly to those upstream specifications in a non-normative note.

In addition to considering unifying the phrasing with Annex B, I want to suggest that you consider unifying it with ECMA-402's use of normative optional. I think we can continue to iterate on wording as this specification moves towards Stage 4, though, so I'll merge this PR as is.

Looking at the rendered spec, it's not very clear whether being "normative optional" groups with cleanupSome or unregister... We should probably iterate on this.