Disable guest checkout for digital download products on cart

add_filter( 'pre_option_woocommerce_enable_guest_checkout', function( $value ) {

	// Require Cart
	if( empty( WC()->cart ) ) {
		return $value;
	}

	// Loop Cart Items
	foreach( WC()->cart->get_cart() as $item ) {

		// Check Variable Product Line Item
		if( ! empty( $item['variation_id'] ) ) {

			// Check If Downloadable
			$product = wc_get_product( $item['variation_id'] );
			if( $product->is_downloadable() ) {
				return 'no';
			}

		}

		// Check Non Variable Line Item
		if( empty( $item['product_id'] ) ) {
			continue;
		}

		// Check If Downloadable
		$product = wc_get_product( $item['product_id'] );
		if( $product->is_downloadable() ) {
			return 'no';
		}

	}

	// Return
	return $value;

} );