I have did tried to use the code from this answer:
How to get minimum order amount for free shipping in woocommerce
But it return a NULL
Maybe it will help someone. You can create new free shipping object and get from it min_amount. I think it's simpler than in @LoicTheAztec answer.
if ( class_exists( 'WC_Shipping_Free_Shipping' ) ) {
$free_shipping = new WC_Shipping_Free_Shipping();
$min_amount = $free_shipping->min_amount;
echo $min_amount;
}