Hi there,
I added a filter in the plugin to satisfy your need.
Please download the file here and replace it at following path:
wp-content/plugins/yith-woocommerce-compare/includes/class.yith-woocompare-frontend.php
Then add following code in the file functions.php of your theme.
add_filter( 'yith_woocompare_redirect_status', 'yith_woocompare_set_redirect_status' );
if( ! function_exists( 'yith_woocompare_set_redirect_status' ) ){
function yith_woocompare_set_redirect_status( $status ){
$status = 301;
return $status;
}
}
All changes applied to the file I shared with you will be included in the next official release too.