Not necessarily. There are other restrictions we could possibly place on errors that allow them to be propagated without requiring the full expressivity of general existentials. Typed throws might be a good feature in and of itself, but requiring its use in embedded Swift could be a pretty severe dialectization wedge. Our guidance for long-term stable APIs will likely still be to recommend untyped errors to allow for library evolution, and we'd want libraries that intend to target both desktop and embedded Swift to follow that guidance without having to diverge their APIs.