The xemacs pkg of Reftex is version 4.21.
There is at least version 4.34 available as a stand alone package. More recent
versions are part of GNU emacs and may or may not run under Xemacs.
However I recommend to upgrade at least to 4.34.
I am not sure who is the actual maintainer. The Changelog suggests that
it is basically orphaned. I would volunteer, but would need write access
to that part of bitbucket.
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