OCLC has announced <http://www.oclc.org/developer/news/2015/change-to-xid-services.en.html> that their xID services, including xISBN, will be discontinued on 15 March 2016. Consequently, code that uses it should be removed. The affected feature (the "Editions" tab in the public catalog record details view) is turned on via the FRBRizeEditions, XISBN, and OCLCAffiliateID system preferences and is implemented in the C4::XISBN module. Note that the feature can also use ThingISBN (from LibraryThing) and Syndetics as data providers, so we're not in the position of having to remove it entirely -- just use of OCLC xISBN.
It might live a little longer: http://www.oclc.org/developer/news/2016/xid-retirement-delayed.en.html
Indeed. I'll mark this bug invalid for now.