@evan With Usenet/NNTP every post and reply had a Subject: line like email.
If you wanted to go on a tangent you could edit the Subject:.
For example if it was
Subject: X
you would change it to
Subject: Y (was X)
When threaded newsreaders were created, they would detect these changed subject and although they presented them w/ the original thread, they would mark them as different.
There would be an ASCII art thread diagram with the original Subject: marked as [1], and tangents as [2] etc