Once you publicly post something, or even post something privately to someone else, it’s not entirely yours anymore, in an important sense it’s theirs.

Yes Usenet used to gave a spool lifetime, but that was clearly variable and there were no guarantees about it. Anybody could set up a mirror and frequently did. Also anyone could copy out content to other media and there was no expectation that they couldn’t do that.

We are all responsible for what we post to other people and in some cases also the effect it has on them. Consider bullying, abuse, criminal conspiracy, the record of receiving a message belongs to the receiver not the sender.