CMB2 only on front page - Use a custom 'show_on_cb' function to only show the CMB2 metabox when editing the front page. https://www.damiencarbery.com/2020/02/use-cmb2-on-front-page/

cmb2-callback-front-page.php

<?php
function dcwd_show_cmb2_on_front_page( $cmb ) {
	// Get ID of page set as front page, 0 if there isn't one
	$front_page = get_option( 'page_on_front' );

	// There is a front page set  - are we editing it?
	return $cmb->object_id() == $front_page;
}

cmb2-metabox-front-page-only.php

<?php
/*
Plugin Name: CMB2 only on front page
Plugin URI: https://www.damiencarbery.com/2020/02/use-cmb2-on-front-page/
Description: Use a custom 'show_on_cb' function to only show the CMB2 metabox when editing the front page.
Author: Damien Carbery
Version: 0.2
*/


add_action( 'cmb2_admin_init', 'dcwd_home_page_listings' );
function dcwd_home_page_listings() {
	$prefix = '_dcwd_';

	$cmb = new_cmb2_box( array(
		'id'           => $prefix . 'home_page_listings',
		'title'        => 'Column 2 Settings',
		'object_types' => array( 'page' ),
		'context'      => 'after_editor',
		'show_names'   => true,
		'show_on_cb' => 'dcwd_show_cmb2_on_front_page',
	) );

	$cmb->add_field( array(
		'name'    => 'Column 2 title',
		'desc'    => 'Enter the text to be shown above the posts in column 2.',
		'id' => $prefix . 'col2title',
		'type'    => 'text',
	) );
	$cmb->add_field( array(
		'name' => 'Number of posts to show',
		'desc' => 'Numbers only',
		'id'   => $prefix . 'col2count',
		'type' => 'text',
		// Ensure that only numbers are entered.
		'attributes' => array(
			'type' => 'number',
			'pattern' => '\d*',
		),
	) );

}

function dcwd_show_cmb2_on_front_page( $cmb ) {
	// Get ID of page set as front page, 0 if there isn't one
	$front_page = get_option( 'page_on_front' );

	// There is a front page set - are we editing it?
	return $cmb->object_id() == $front_page;
}


// Verify that CMB2 plugin is active.
add_action( 'admin_notices', 'verify_cmb2_active' );
function verify_cmb2_active() {
	if ( ! defined( 'CMB2_LOADED' ) ) {
		$plugin_data = get_plugin_data( __FILE__ );
		$plugin_name = $plugin_data['Name'];
?>
<div class="notice notice-warning is-dismissible"><p>Plugin <strong><?php echo $plugin_name; ?></strong> requires <a href="https://wordpress.org/plugins/cmb2/">CMB2 plugin</a>.</p></div>
<?php
		//error_log( 'CMB2 is not active.' );
	}
}