The IMAGE_RESERVED_CUSTOM_PROPERTIES setting can be found in the documentation, in the translation strings and in the upgrade check script, but is not actually used anywhere:
$ git grep IMAGE_RESERVED_CUSTOM_PROPERTIES
doc/source/configuration/settings.rst:IMAGE_RESERVED_CUSTOM_PROPERTIES
doc/source/locale/en_GB/LC_MESSAGES/doc-configuration.po:msgid "IMAGE_RESERVED_CUSTOM_PROPERTIES"
doc/source/locale/en_GB/LC_MESSAGES/doc-configuration.po:msgstr "IMAGE_RESERVED_CUSTOM_PROPERTIES"
doc/source/locale/id/LC_MESSAGES/doc-configuration.po:msgid "IMAGE_RESERVED_CUSTOM_PROPERTIES"
doc/source/locale/id/LC_MESSAGES/doc-configuration.po:msgstr "IMAGE_RESERVED_CUSTOM_PROPERTIES"
openstack_dashboard/management/commands/upgrade_check.py: 'IMAGE_RESERVED_CUSTOM_PROPERTIES',
It was removed from Horizon in commit 3c2ce866df58eccb4eef44ae766997236c074ff1 .
Should IMAGE_RESERVED_CUSTOM_PROPERTIES be removed from the documentation, or should it make its way back in the code?
See also: https://bugs.launchpad.net/horizon/+bug/1830301
