Create a custom message email variable for PMPro Approvals Add On.

pmpro-approvals-custom-message-data.php

<?php
/**
 * This recipe adds a custom message per membership level to PMPro_Approvals email data.
 *
 * This recipe assumes that the !!custom_approved_message!! variable was added to the
 * Approvals - Approved Email template, and
 * the !!custom_denied_message!! variable was added to the
 * Approvals - Denied Email template.
 *
 * 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/
 */

// # APPROVED #
// Data for the !!custom_approved_message!! email variable in the Approvals - Approved Email template.
function my_pmpro_approvals_member_approved_email_data( $data, $member ) {
	$level_id              = $data['membership_id'];
	$membership_level_name = $data['membership_level_name'];

	// Let's match the approval level ID and create a custom message for that level.
	switch ( $level_id ) {
		case '1':
			$custom_approved_message = '<p>' . esc_html( 'You are now a member of the ' . $membership_level_name . ' level.' ) . '</p>';
			break;
		case '2':
			$custom_approved_message = '<p>' . esc_html( 'Custom approved message for the ' . $membership_level_name . ' level.' ) . '</p>';
			break;

		default:
			$custom_approved_message = '<p>' . esc_html( 'Default message here for other approval levels not specified above' ) . '</p>';
			break;
	}

	$data['custom_approved_message'] = $custom_approved_message;

	return $data;
}
add_filter( 'pmpro_approvals_member_approved_email_data', 'my_pmpro_approvals_member_approved_email_data', 10, 2 );

// # DENIED #
// Data for the !!custom_denied_message!! email variable in the Approvals - Denied Email template.
function my_pmpro_approvals_member_denied_email_data( $data, $member ) {
	$level_id              = $data['membership_id'];
	$membership_level_name = $data['membership_level_name'];

	// Let's match the approval level ID and create a custom message for that level.
	switch ( $level_id ) {
		case '1':
			$custom_denied_message = '<p>' . esc_html( 'Custom message for the ' . $membership_level_name . ' level.' ) . '</p>';
			break;
		case '2':
			$custom_denied_message = '<p>' . esc_html( 'Custom message for the ' . $membership_level_name . ' level.' ) . '</p>';
			break;

		default:
			$custom_denied_message = '<p>' . esc_html( 'Default message here for other approval levels not specified above' ) . '</p>';
			break;
	}

	$data['custom_denied_message'] = $custom_denied_message;

	return $data;
}
add_filter( 'pmpro_approvals_member_denied_email_data', 'my_pmpro_approvals_member_denied_email_data', 10, 2 );

Originally, I missed the step you mentioned in the comments to add !!custom_approved_message!! to the email template, but worked great after doing so!