If trxn_id is present, then show it.
The variable is always assigned in
https://github.com/civicrm/civicrm-core/blob/
2ad38a735f7b26daf99d7b63eb413c6b69f780cb/CRM/Contribute/Form/Contribution/ThankYou.php#L212
So we don't need more (deprecated) conditions
{if $receive_date}
{ts}Date{/ts}: <strong>{$receive_date|crmDate}</strong><br />
{/if}
- {if $contributeMode ne 'notify' and $is_monetary and ! $is_pay_later and $trxn_id}
+ {if $trxn_id}
{ts}Transaction #{/ts}: {$trxn_id}<br />
{/if}
{if $membership_trx_id}