James Clark has suggested that his XSLT processor, XT, should quietly fade away in favor of other XSLT processors.

The editor of the XSLT 1.0 specification, whose XT processor still has a significant user base, has stepped back from developing the tool:

My main goal in writing XT was to support the development of the XSLT spec ... With the publication of the Rec, this goal has been achieved, and consequently I haven't done significant work on it since then.

Clark, citing that he saw no point in maintaining multiple independent Java XSLT processors, indicated that he may write up the internals of XT if other implementors wished to learn from its performance.

If you're an active user of XT, all is not lost. XT is being actively supported by a community of users at 4xt.org. 4xt.org has released a maintenance plan for further development of the software.