It is not uncommon for a valid iTunes reference to become invalid. An example is here:
The iTunes reference on this release is not longer valid. I considered removing the reference, but that reference has historical meaning. The ID used, although no longer active, has reference embedded to media files from anyone who acquired it.
My question would be how to handle these references. I do not think complete removal is wise, but it is also not valid as a reference anymore either. Meaning, when there is a reference for a release, the link is expected to work, otherwise I would assume the common thought to be remove the reference and replace it with something valid.