Given that there's a note in the 1.3 spec about the no-longer-restricted "XML" prefix in the XML spec, I figured we must have discussed this in a meeting before, so out of curiosity I went looking through the aggregated minutes:
From 19 Aug 2014:
2. New item: XML mention domain
Nancy gave overview; after her talk on this new domain in the 1.3 webinar, Sahdor Kekesi, who'd attended the webinar, wrote to ask about how we can use the new elements that start with 'xml', when the XML spec reserves the use of that string at the beginning of an SML element. We appear to be doing something illegal in XML.
From 26 Aug 2014:
7. XML mention domain revisited
[More research has revealed that the reservation of the 'xml' string is no longer in the XML spec, so we can avoid having to make any changes.]
Kris; so, we were unaware of fact that it was no longer a problem.
Don; in the future, we need to be cautious; we can go ahead with using the 'xml', but we need to explain it, for people who remember the reservation and don't know it's gone away.
Michael; we could include a note and link in the xmlmention topic
Robert; we don't want too much, just a note and/or link in the parent xmlmention topic.
ActionItem; Don will draft the note to be put into topic explaining the issue.