Re: Should the XML "recommendation" have an expiration date?

They have to be reviewed every 5 years (3 years for Technical Reports) to decide whether to keep them (or revise them or convert them to a different class). Otherwise they will be withdrawn.  After withdrawal, they can be reinstated again after a full vote.

"Withdrawn" does not mean that they are nullified, as if they never existed, so it is completely possible for some user to adopt a withdrawn standard. It just means there is no agreement (or interest) from the ISO members that the publication's market is not now better served by having the publication on ISO's books under active maintenance than not. So a withdrawal does not necessarily reflect poorly on the technology: a technology may have a steady community, but one not available to participate in standardization efforts, for example.  Or times and markets may have changed, or better technologies come along.

Cheers
Rick

Every International Standard and other deliverable published by ISO or jointly with IEC shall be subject

to systematic review in order to determine whether it should be confirmed,

revised/amended, converted to another form of deliverable, or withdrawn,
http://isotc.iso.org/livelink/livelink?func=ll&objId=4230452&objAction=browse&sort=subtype