Add has-\d-columns class to columns blocks after WordPress 5.3 update.

has-d-columns.php

<?php
/**
 * Add has-\d-columns class to columns blocks after 5.3 update.
 * 
 * @param	string	$block_content The block content about to be appended
 * @param	array	$block The full block, including name and attributes
 * @return	string The updated block content
 */
add_filter( 'render_block_core/columns', function( $block_content, $block ) {
	$columns_count = count( $block['innerBlocks'] );
	
	// Check if the columns block already has a has-\d-columns class.
	if ( preg_match( '/<div class="(?:.* )?(wp-block-columns (?:.*)has-' . $columns_count . '-columns|has-' . $columns_count . '-columns (?:.*)wp-block-columns)(?:.*)?"(?:.*)>/', $block_content ) ) {
		return $block_content;
	}
	
	// Add the class to the markup.
	$block_content = preg_replace( '/<div class="(.* )?wp-block-columns((?:.* ?)?)"(.*)>/', sprintf(
		'<div class="$1wp-block-columns has-%d-columns $2"$3>',
		$columns_count
	), $block_content, 1 );
	
	return $block_content;
}, 10, 2 );