Like most social rules, it is an unspoken one, so obviously not explicitly written down.

That being said, this particular piece of etiquette harkens all the way back to the classic web rule of "no necroing", which is still a common unspoken rule in one form or another in many parts of the Web.

If you want to reference an old webpage, that's fine, but in that case it's best to not post it like it's a new webpage, which is what this does.

No notice or even a single mention of how it's over 10 years old.

You don't like the unspoken rule, that's fine. But don't whine about it or act shocked when people complain.