I suspect that this question was put On Hold after your commentyour comment:
We ended up writing the CartoCSS from scratch. It didn't take that long, and our designer was more happy writing the CSS, than he ever was fiddling around with the XML.
When I read that I think "Oh, he no longer has a requirement for a SLD to Carto CSS".
If that was @Iant's thinking too, then I think it would have been better to use our recently modified custom close reason from Improving wording of "non-reproducible" custom close reason? because these words from that would seem to apply:
Changes to ... the asker's circumstances may have rendered the question obsolete ...
From the same Meta GIS SE Q&A there is additional advice which I think will apply to the question that you are asking about:
the mechanism to re-open a question is already there for those cases where anyone may use it (or any close reason) overzealously
If you are still looking for "a SLD to Carto CSS" then I think you should revise your question to incorporate any relevant content from the comments and stress that you are still looking for a solution despite your workaround. That should trigger its re-entry into the Re-open queue, and if not, then flag a moderator to review it.