Under both version 0.18 and version 0.18.1 of Docutils, rst2html5.py does not recognize configuration settings in an [html writers] section in docutils.conf. This appears to be due to commit 8722, in which _html_base.py was edited to no longer include "html writers" in its config_section_dependencies, instead moving the value to config_section; while html4css1/__init__.py was updated in the same commit to include "html writers" in its config_section_dependencies, html5_polyglot/__init__.py did not get the same treatment.
Fixed in [r8967].
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Commit: [r8967]
Release 0.19 (2022-07-05) fixes the issue.
Thank you for the report.