The List-Unsubscribe header is an optional chunk of text that email
publishers and marketers can include in the header portion of the
messages they send. Recipients don't see the header itself, they see an
Unsubscribe button they can click if they would like to automatically
stop future messages.

Refer -

It is very important nowadays and used by many ISP's and big email providers.

Would like to see this header added in WHMCS.