Add support for serial commas to Co-Authors Plus WordPress plugin.

serial-commas-CAP.php

<?php

/**
 * Add support for serial commas to Co-Authors Plus WordPress plugin.
 * Filters coauthors_default_between_last to support serial commas (oxford comma) for 3 or more coauthors
 */
add_filter( 'coauthors_default_between_last', function( $between_last ) {
	if ( ' and ' === $between_last && count( get_coauthors() ) > 2 ) {
		$between_last = ', and ';
	}
	return $between_last;
});