Remove the text "Your %s membership is now active" from confirmation message if order does not have "success" status.

my_pmpro_confirmation_message_not_success.php

<?php

// Copy from below here...

/*
 * Remove the text "Your %s membership is now active" from confirmation message
 * if order does not have "success" status.
 *
 * 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_confirmation_message_not_success( $message, $invoice ) {
	if ( empty( $invoice ) ) {
		return $message;
	}

	if ( $invoice->status !== 'success' ) {
		global $current_user;
		$message = str_replace( ' Your ' . $current_user->membership_level->name . ' membership is now active.', '', $message );
	}
	return $message;
}
add_filter( 'pmpro_confirmation_message', 'my_pmpro_confirmation_message_not_success', 10, 2 );