/usr/share/apache2/error/ is in apache2-data 2.4.18-2ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/share/apache2/error/HTTP_BAD_GATEWAY.html.var
/usr/share/apache2/error/HTTP_BAD_REQUEST.html.var
/usr/share/apache2/error/HTTP_FORBIDDEN.html.var
/usr/share/apache2/error/HTTP_GONE.html.var
/usr/share/apache2/error/HTTP_INTERNAL_SERVER_ERROR.html.var
/usr/share/apache2/error/HTTP_LENGTH_REQUIRED.html.var
/usr/share/apache2/error/HTTP_METHOD_NOT_ALLOWED.html.var
/usr/share/apache2/error/HTTP_NOT_FOUND.html.var
/usr/share/apache2/error/HTTP_NOT_IMPLEMENTED.html.var
/usr/share/apache2/error/HTTP_PRECONDITION_FAILED.html.var
/usr/share/apache2/error/HTTP_REQUEST_ENTITY_TOO_LARGE.html.var
/usr/share/apache2/error/HTTP_REQUEST_TIME_OUT.html.var
/usr/share/apache2/error/HTTP_REQUEST_URI_TOO_LARGE.html.var
/usr/share/apache2/error/HTTP_SERVICE_UNAVAILABLE.html.var
/usr/share/apache2/error/HTTP_UNAUTHORIZED.html.var
/usr/share/apache2/error/HTTP_UNSUPPORTED_MEDIA_TYPE.html.var
/usr/share/apache2/error/HTTP_VARIANT_ALSO_VARIES.html.var
/usr/share/apache2/error/README
/usr/share/apache2/error/contact.html.var
/usr/share/apache2/error/include/
/usr/share/apache2/error/include/bottom.html
/usr/share/apache2/error/include/spacer.html
/usr/share/apache2/error/include/top.html