Not
A form the DSL accepts and the library does not yet carry (G.9 row 11).
Thrown at the declaration, not at the eventual use. A modeler who writes something legal should learn immediately that it is not yet supported and which milestone carries it — not run a study and find the value missing. When the milestone lands, the throw is replaced by the implementation and nothing else changes.
milestone is the weak part of this message and is known to be. section is durable — it names where the form is specified — while a milestone label is a project-plan coordinate that means nothing to a reader outside the project and goes stale when the plan is redrawn. It has gone stale twice: once when §7.1.1 moved work between milestones, and once when a step was reported complete while still carrying a refusal that pointed at it. The label is kept because during development it is the most useful thing to tell an implementer, and it should not survive into a release — at which point every remaining use of this exception is itself a release blocker, so the question resolves itself.