As we found no evidence of xmp records being used, there was no case for keeping the xmp properties value as under-implemented, or even keeping it as deprecated (only the onix property is used). It should be safe to remove this from validation.

The acquire rel value was also removed from the vocabulary, but it should be left valid because it was defined in the Previews spec. I think we have some partial validation support for previews, but it's probably not worth the effort of making it valid only in those types of publications - just leave it as an acceptable rel value.

See w3c/epub-specs#2489