I think we should remove the required fields stac_version and stac_extensions from ItemCollection.
stac_version contains the stac-spec version in all other specs, but ItemCollection is not even defined in stac-spec. So what to include there? I think the ItemCollection schema in API responses is implicitly clear through the Conformance classes and thus should be handled similarly to for example the "Collection Collection" (i.e. /collections) for example. There's also no stac_version etc. included.