Generally anything that is not time sensitive, either because it is about a generic or static topic, or because the author makes regular updates if it is not static. EG: a post on WWII airplanes would be evergreen, as the data is pretty much not going to change at this point. Similarly a regularly updated post on stealth airplane technology could also be evergreen because it evolves with the technology and is presumably always current and relevant.
A post on raising VC funding that was written in 2007 and not updated is not really evergreeen because while it was accurate for the time, it is probably not accurate and relevant in the current funding market.
I could quip that "Here's what Trump tweeted today" is the evergreen content, because you always know what Trump tweets are going to be like. But that wouldn't be a HN-worthy comment, so I won't.
A post on raising VC funding that was written in 2007 and not updated is not really evergreeen because while it was accurate for the time, it is probably not accurate and relevant in the current funding market.