EDItEUR has published advice for any organisation using ONIX 2.1 after the 'sunset' date at the end of December 2014 (only a month or so away).
Note that ONIX 2.1 will not ‘stop working’ on 1st January 2015 – it will still be usable, but will not receive the level of support that it currently has. In particular, documentation will be archived and online DTD / XSD support for XML validation will be removed. This will affect anyone relying on the DTD / XSD files on the EDItEUR website, for example for validation of XML files.
For any organisation using ONIX 2.1 after sunset and relying on the online DTD and XSD files provided by EDItEUR, some active steps to avoid failures in production systems will be necessary.
EDItEUR has published a note describing the workaround you need to put in place to avoid failures in processing ONIX 2.1 files after sunset. The paper can be downloaded here. The workaround is straightforward, but will require some technical skill and administrator access to the relevant computers to implement.