But, why does it matter for all articles? Is it just a blind rule, or can we apply it just when it actually makes sense?
The year in the title means: it is so good, it is worth reposting here.
It has less to do with math, and more to do with the conventions of posting to HN where people expect dates for non-recent links. There's nothing wrong with them otherwise.
The convention exists because for most articles, publication year is an important piece of context. I'm not sure we have to blindly follow the convention when it isn't.
There is an aspect of novelty when it comes to YouTube videos
Something has to define when new math came to be /s