def METHOD_NAME(config: Config) -> list[str]:
    """
    Return the list of all plugin indexes.
    """
    config.setdefault(PLUGIN_INDEXES_KEY, [])
    indexes = get_typed(config, PLUGIN_INDEXES_KEY, list)
    for url in indexes:
        if not isinstance(url, str):
            raise TutorError(
                f"Invalid plugin index: {url}. Expected 'str', got '{url.__class__}'"
            )
    return indexes