Conditional payment gateways by variation shipping status

add_action( 'init', 'disable_cod_for_backorder_variations' );
function disable_cod_for_backorder_variations() {
	if ( current_user_can( 'administrator' ) ) {
		// WP_Query to get all product variations
		$args = array(
			'post_type'      => 'product_variation', // Query for product variations
			'posts_per_page' => - 1,                 // Get all variations
			'post_status'    => 'publish',          // Only published variations
			'fields'         => 'ids',              // Only retrieve variation IDs
		);

		$variation_query = new WP_Query( $args );

		// Retrieve the current excluded products from the database (for COD)
		$excluded_cod_products = get_option( 'alg_wc_cpg_product_excl', array() );

		// If the 'cod' key doesn't exist, initialize it as an empty array
		if ( ! isset( $excluded_cod_products['cod'] ) ) {
			$excluded_cod_products['cod'] = array();
		}

		// Loop through all product variations
		foreach ( $variation_query->posts as $variation_id ) {
			$variation = wc_get_product( $variation_id );

			// Check if the variation is on backorder
			if ( $variation->is_on_backorder() ) {
				// Cast $variation_id to a string explicitly
				$variation_id_str = (string) $variation_id;

				// Add the variation ID (as a string) to the exclusion list for COD if it's not already there
				if ( ! in_array( $variation_id_str, $excluded_cod_products['cod'], true ) ) {
					$excluded_cod_products['cod'][] = $variation_id_str;
				}
			}
		}

		// Ensure all existing IDs in the array are strings (for consistency)
		$excluded_cod_products['cod'] = array_map( 'strval', $excluded_cod_products['cod'] );

		// Save the updated exclusion list into the database
		update_option( 'alg_wc_cpg_product_excl', $excluded_cod_products );
	}
}

Please test this on your staging site first and let me know if it works for you.

Best regards,
WPFactory Support Team