Add custom shortcodes within EE_Event_Shortcodes and EE_Datetime_shortcodes available within the EE Messages system.

tw_custom_messages_shortcodes.php

<?php
/*
Plugin Name: EE Custom Messages Shortcode - Site Specific Plugin
Description: Add custom messages shortcodes for Event Espresso 4 here.
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */
function register_new_tony_shortcodes( $shortcodes, EE_Shortcodes $lib ) {
  
	//Add a shortcode to be used with the EE Datetimes within messages
	if ( $lib instanceof EE_Datetime_Shortcodes ) {
		//Add your shortcode to the add as the key, the value should be a description of the shortcode.
		$shortcodes['[TONYS_DATETIME_SHORTCODE]'] = _('This is Tonys Custom DATETIME Shortcode!');
	}
	//Add a shortcode to be used with the EE Event List within messages
	if ( $lib instanceof EE_Event_Shortcodes ) {
		//Add your shortcode to the add as the key, the value should be a description of the shortcode.
		$shortcodes['[TONYS_EVENT_SHORTCODE]'] = _('This is Tonys Custom EVENT Shortcode!');
	}

	//Return the shortcodes.
	return $shortcodes;
}
add_filter( 'FHEE__EE_Shortcodes__shortcodes', 'register_new_tony_shortcodes', 10, 2 );
function register_new_tony_shortcodes_parser( $parsed, $shortcode, $data, $extra_data, EE_Shortcodes $lib ) {
	
	//Check for the datetime shortcodes as that's were we added the custom shortcode above
	//also check that $data is the expected object (in this case an EE_Datetime)
	if ( $lib instanceof EE_Datetime_Shortcodes  && $data instanceof EE_Datetime ) {
		//Then check for our specific shortcode
		if ( $shortcode == '[TONYS_DATETIME_SHORTCODE]' ) {
			
			//Do whatever you need to do here and return the value for that specific datetime.
			return $data->ID();
		}
	}

	if ( $lib instanceof EE_Event_Shortcodes ) {
	//Then check for our specific shortcode
		if ( $shortcode == '[TONYS_EVENT_SHORTCODE]' ) {
		
			//First check we have an event object.
			$event = $data instanceof EE_Event ? $data : null;

			//if no event, then let's see if there is a reg_obj.  If there IS, then we'll try and grab the event from the reg_obj instead.
			if ( empty( $event ) ) {
				$aee = $data instanceof EE_Messages_Addressee ? $data : NULL;
				$aee = $extra_data instanceof EE_Messages_Addressee ? $extra_data : $aee;

				$event = $aee instanceof EE_Messages_Addressee && $aee->reg_obj instanceof EE_Registration ? $aee->reg_obj->event() : NULL;
			}
			
			//Check we do now actually have the event object.
			if ( !empty( $event ) ) {
				//Do whatever you need to do using the event object, which is now $event and return.
				return $event->ID();
			}
		}
	}

	//If not within the correct section, or parsing the correct shortcode,
	//Return the currently parsed content.
	return $parsed;
}
add_filter( 'FHEE__EE_Shortcodes__parser_after', 'register_new_tony_shortcodes_parser', 10, 5 );

References:

https://eventespresso.com/topic/custom-shortcodes-in-messages/
https://eventespresso.com/topic/venue-against-each-date/#post-177488
https://eventespresso.com/topic/can-i-use-shortcodes-from-other-message-types/
https://eventespresso.com/topic/payment-tx-id-in-event-admin-emails/