Just tested 5.8.1 and looks like issue still is not resolved.

+ /usr/bin/python3 setup.py build_sphinx -b man --build-dir build/sphinx
/usr/lib/python3.8/site-packages/setuptools/command/easy_install.py:160: EasyInstallDeprecationWarning: easy_install command is deprecated. Use build and pip and other standards-based tools.
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/command/install.py:34: SetuptoolsDeprecationWarning: setup.py install is deprecated. Use build and pip and other standards-based tools.
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/dist.py:738: UserWarning: Usage of dash-separated 'author-email' will not be supported in future versions. Please use the underscore name 'author_email' instead
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/dist.py:738: UserWarning: Usage of dash-separated 'description-file' will not be supported in future versions. Please use the underscore name 'description_file' instead
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/dist.py:738: UserWarning: Usage of dash-separated 'description-content-type' will not be supported in future versions. Please use the underscore name 'description_content_type' instead
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/dist.py:738: UserWarning: Usage of dash-separated 'home-page' will not be supported in future versions. Please use the underscore name 'home_page' instead
  warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/dist.py:738: UserWarning: Usage of dash-separated 'python-requires' will not be supported in future versions. Please use the underscore name 'python_requires' instead
  warnings.warn(
running build_sphinx
[pbr] Generating AUTHORS
[pbr] AUTHORS complete (0.0s)
[pbr] Sphinx 1.6 added native support for specifying multiple builders in the '[sphinx_build] builder' configuration option, found in 'setup.cfg'. As a result, the '[sphinx_build] builders' option has been deprecated and will be removed in pbr 4.0. Migrate to the 'builder' configuration option.
Running Sphinx v4.5.0
[openstackdocstheme] version: 2.4.0
[openstackdocstheme] connecting html-page-context event handler
making output directory... done
Creating file /home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/reference/api/pbr.rst.
Creating file /home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/reference/api/pbr.cmd.rst.
Creating file /home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/reference/api/pbr.hooks.rst.
Creating file /home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/reference/api/modules.rst.
[openstackdocstheme] using theme from /usr/lib/python3.8/site-packages/openstackdocstheme/theme
fatal: not a git repository (or any parent up to mount point /home/tkloczko)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
[openstackdocstheme] cannot find git top directory, assuming "."
building [mo]: targets for 0 po files that are out of date
building [man]: all manpages
updating environment: [new config] 15 added, 0 changed, 0 removed
reading sources... [100%] user/using
/home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/user/history.rst:1: CRITICAL: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'ChangeLog'.
/home/tkloczko/rpmbuild/BUILD/pbr-5.8.1/doc/source/user/releasenotes.rst:5: CRITICAL: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'RELEASENOTES.rst'.
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
writing... python-prb.3 { user/index user/features user/using user/packagers user/semver user/compatibility user/releasenotes user/history reference/index reference/api/modules reference/api/pbr reference/api/pbr.cmd reference/api/pbr.hooks contributor/index } done
build succeeded, 2 warnings.