Setting send classification transactional generates an email that includes list-help header that links subscription center page. I do not want this header in my email. How can I remove it?
I noticed this as well.
tl;dr: It cannot be removed.
This was introduced in this release: https://help.salesforce.com/s/articleView?id=sf.mc_rn_january_2021_es_one_click_unsub1.htm&type=5
...where Salesforce introduced the http header, not just the reply based header for list-unsubscribe. They state it is for commercial emails.
Contrary to what the release notes say, this WAS implemented also in Transactional emails, which results in having an unsubcription header where it "does not belong" (after all, transactional emails cannot be unsubscribed from).
The linked documentation in the above release note states the opposite (it IS in transactional emails) and discusses that this cannot be changed:
NOTE Because Request for Comments (RFC) 8058 regulations for one-click unsubscribe require acceptance of POST requests, our List-Unsubscribe headers can’t be disabled or altered. Directing one-click unsubscribes to landing pages deviates from RFC 8058 regulations and isn’t supported.
...and describes a "fig leaf" (my interpretation) explanation why this would be useful:
List-Help Header for Transactional Emails
Marketing Cloud includes the List-Help header in transactional classification emails. If the subscriber can’t determine who is sending the email and why from the body of an email, they can use the link in the List-Help header. The List-Help header allows the subscriber to view the Subscription Center page, which includes details about the sender and their sending policies.