maybe_redirect_404()

Source

function maybe_redirect_404() {
	if ( is_main_site() && is_404() && defined( 'NOBLOGREDIRECT' ) ) {
		/**
		 * Filters the redirect URL for 404s on the main site.
		 *
		 * The filter is only evaluated if the NOBLOGREDIRECT constant is defined.
		 *
		 * @since 3.0.0
		 *
		 * @param string $no_blog_redirect The redirect URL defined in NOBLOGREDIRECT.
		 */
		$destination = apply_filters( 'blog_redirect_404', NOBLOGREDIRECT );

		if ( $destination ) {
			if ( '%siteurl%' === $destination ) {
				$destination = network_home_url();
			}

			wp_redirect( $destination );
			exit;
		}
	}
}

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