Following #2563, we'll have no strong reason to keep `myst-spec-ext` around. We can deprecate `myst-spec-ext` by: 1. Merging the types into `myst-spec` 2. Authoring an MEP to reflect the spec update 3. Marking `myst-spec` as deprecated ## Definition of done - [ ] `myst-spec-ext` is published as deprecated
