Source

File: wp-includes/feed.php

function get_wp_title_rss( $deprecated = '–' ) {
	if ( '–' !== $deprecated ) {
		/* translators: %s: 'document_title_separator' filter name. */
		_deprecated_argument( __FUNCTION__, '4.4.0', sprintf( __( 'Use the %s filter instead.' ), '<code>document_title_separator</code>' ) );
	}

	/**
	 * Filters the blog title for use as the feed title.
	 *
	 * @since 2.2.0
	 * @since 4.4.0 The `$sep` parameter was deprecated and renamed to `$deprecated`.
	 *
	 * @param string $title      The current blog title.
	 * @param string $deprecated Unused.
	 */
	return apply_filters( 'get_wp_title_rss', wp_get_document_title(), $deprecated );
}