Currently, the extra names are passed to packaging.requirements.Requirement.marker.evaluate as given to %pyproject_buildrequires -x. As a result, nonexisiting extras don't cause any errors/warnings. We should parse the available extras from package METADATA/[project] and issue warnings (perhaps errors later) for nonexisting extras. When we know all the available extras, we could even treat -x arguments as globs (and allow -x '*' etc.).
PR: https://src.fedoraproject.org/rpms/pyproject-rpm-macros/pull-request/534