Thanks to Peter Brett for pointing this out to me the other day.  The latest revision of the Unicode UTS#18 covering regular expressions has some significant changes.  In particular, the level 3 specification covering tailoring has been dropped.

The current version of the UTS is available at https://www.unicode.org/reports/tr18/tr18-21.html.

Diff marks relative to the prior version can be viewed at https://www.unicode.org/reports/tr18/tr18-20.html.

These changes appear to have been approved at UTC meeting #162 in May, 2020.  Minutes are available at https://www.unicode.org/L2/L2020/20015.htm; search for "UTS #18".  The relevant papers covering the changes appear to be:

• L2/19-364R2, Feedback on Proposed Update UTS #18, Markus Scherer, 2010-10-08.
  
• L2/20-017, Comments on Public Review Issues.
• See the link to issue 404.
  
• L2/20-032, Document for review, Mark Davis and Andy Heninger, 2019-11-08.
  
• L2/20-051, Response to feedback on UTS18, Mark Davis, 2020-01-14.

The upshot for us is that, when we get back around to looking at std::regex replacements, we won't have to spend time worrying about support for tailoring.

Tom.