It would be very helpful if metafields could be exposed on Collection also in the storefront api. All the infrastructure seems to be in place for this to happen as it is exposed on Product and ProductVariant.
The metafields can already be made visibile via the metafieldStorefrontVisibilityCreate by setting the owner type as COLLECTION. However the metafields is not exposed at all in the storefront api.
Would be really great if someone could enable that. I can't really see any downsides given metafields would not be visible by default, and from all I could tell it should be pretty quick to do.
It is a feature request we are aware of and will investigate adding. The best place to watch will be the changelog. Thanks!