I'm not sure I agree about necroposting. I find it tends to increase the noise level more than simply creating a new thread about the topic in some cases, as people see a post with tons of comments and don't realize that many of those comments are out of date and no longer accurate/relevant.

There's obviously a line to draw somewhere. You don't want 50 posts on the same topic clogging the top of the feed, but zombie threads where the first 90% is out of date or irrelevant are also bad. Locking a thread after it has been idle for a week or two seems like a reasonable compromise to me.

And completely unreasonable to me. Even on reddit the cutoff is... 6 months ? People not realizing that it's a necropost is either a failure of the poster not declaring it as such, or of the forum software - see how Discourse auto-highlights necroposting !

How is a poster supposed to go back to the 5 year old thread starting comment and change it to note that the post is out of date?

Good forum software makes it less of a sin, but that's pretty rare on the Internet. This isn't the Usenet where people's clients could be fully featured on every topic.

Usenet would have it worse, because there you can choose your software yourself (and pick one without that feature)?

BTW, I prefer decentralized protocols like Usenet, there are plenty of features that would be unthinkable to leave out of "modern" clients, this could be one of them !