Display stock availability for unmanaged stock in WooCommerce - Admin - Products listing.

functions.php

// Add custom column to the products admin list
add_filter( 'manage_edit-product_columns', 'add_custom_stock_status_column' );

function add_custom_stock_status_column( $columns ) {
	$new_columns = array();
	foreach ( $columns as $key => $value ) {
		$new_columns[ $key ] = $value;
		if ( $key === 'sku' ) { // Move after SKU column
			$new_columns['custom_stock_status'] = 'Disponibilitate';
		}
	}

	return $new_columns;
}

// Display stock status in the custom column
add_action( 'manage_product_posts_custom_column', 'show_custom_stock_status_column', 10, 2 );

function show_custom_stock_status_column( $column, $postid ) {
	if ( $column === 'custom_stock_status' ) {
		$product = wc_get_product( $postid );
		if ( $product->is_in_stock() ) {
			echo '<mark class="instock">Disponibil</mark>';
		} elseif ( $product->is_on_backorder() ) {
			echo '<mark class="onbackorder">Backorder</mark>';
		} else {
			echo '<mark class="outofstock">Indisponibil</mark>';
		}
	}
}