in docutils/parsers/rst/include/README.txt:
http://docutils.sf.net/docs/ref/rst/substitutions.html is broken. New link is: https://docutils.sourceforge.io/docs/ref/rst/definitions.html
The new hyperlink title is instead of reStructuredText Standard Substitution Definition Sets => reStructuredText Standard Definition Files
Fixed in r8561. Thank you for reporting.
Fixed in Docutils 0.17.
Thanks again for the report.