[stubish]

Forwarding an email should strip this header, probably along with most of the other irrelevant ones potentially containing sensitive information the user isn't aware of. Forwarding an email with GMail only keeps the From, To, Date and Subject headers.

[kragen]

i feel like if we're talking about a header the user isn't aware of, most users probably won't be able to use it to unsubscribe either

[stubish]

End users won't use the header. Email clients will use the header when you hit the report spam or unsubscribe buttons they will display.

[calfuris]

Users don't need to be aware of the header, they just need to use a client that knows what to do with it.