Set the classic editor block as the default block when creating a new post.

gutenberg_classic_editor.php

<?php
/**
 * Plugin Name: Classic Editor First
 * Plugin URI: https://prothemedesign.com
 * Description: Set the default post block to use the classic editor block.
 * Author: Ben Gillbanks
 * Version: 1.0
 * Author URI: https://prothemedesign.com
 */

/**
 * Modify post type arguments to add default post type templates.
 *
 * @param  array  $args      The default post type arguments.
 * @param  string $post_type The post type for the current request.
 * @return array             Modified arguments including the new template properties.
 */
function my_theme_post_type_arguments( $args, $post_type ) {

	// Only apply changes to the specified post type.
	// You could remove this to apply the change to all post types, or change it
	// to filter selected post types.
	if ( 'post' !== $post_type ) {
		return $args;
	}

	/**
	 * Adds a template property to the specified post type arguments.
	 *
	 * You can get a list of available blocks by entering the following js
	 * command in the console window in your browser.
	 * wp.blocks.getBlockTypes()
	 *
	 * The output of this command also shows the available attributes for setting defaults.
	 *
	 * @var array
	 */
	$args['template'] = array(
		array( 'core/freeform' ),
	);

	return $args;

}

add_filter( 'register_post_type_args', 'my_theme_post_type_arguments', 20, 2 );

Nice, simple code. Thanks!
Though I think there is $args missing in first return. Line 23 should probably be:

if ( 'post' !== $post_type ) {
	return $args;
}

Ah ha - good catch. Thanks for pointing that out. I've updated the code.