rel-in-reply-to

 rel=in-reply-to  was a rel value for indicating the destination of a link is an original post that the current page is a reply to; because of its page-scoped rel-limitation as well as lack of consuming code support, you should use u-in-reply-to in addition or instead.

Why not
Currently there is no consuming code or application that depends exclusively on rel=in-reply-to, therefore there is no specific reason why you should publish it.

How to consume
Since there is no critical mass of consuming code or applications that consume rel=in-reply-to, it is unclear if there is any value in attempting to consume it.