Set specific levels to send expiration warning emails to.

my_pmpro_send_expiration_warning_email_level_filter.php

<?php

// Copy from below here...

/**
 * Set specific levels to send expiration warning emails to.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_send_expiration_warning_email_level_filter( $send_email, $user_id ) {
	$levels_to_email = array( 1 );
	return pmpro_hasMembershipLevel( $levels_to_email, $user_id );
}
add_filter( 'pmpro_send_expiration_warning_email', 'my_pmpro_send_expiration_warning_email_level_filter', 10, 2 );