I don’t think there is any way to do this through the Discourse search page. I can see how having a filter to search for deleted posts could be useful for site admins though.
Data Explorer queries can be used to return deleted topics and posts. Something like the query below could help:
--[params]
-- string :term
SELECT
id AS topic_id
FROM topics
WHERE deleted_at IS NOT NULL
AND title LIKE CONCAT('%', :term, '%')
For site’s on our hosting, you can also contact our support team and we’ll find the topic for you.