There is, as others have pointed out, a warning that you're about to post on a thread that's old and then for the next few posts, there's an alert that this is a zombie thread, so hopefully people are aware this is the case at least and can decide to start a new thread instead if they prefer.
@HebeMumsnet that doesn't show if you are only reading hte thread. You can read hundreds f posts before you get to the end of the thread to post, and only then realise it is a zombie thread.
This also happens when someone finds a thread asking a vaguely similar question to theirs, adds their question after 500 posts, and then everyone answers the 8 year old question relating to a child who has now left home, got married and had kids, rather than the new question at the end.
The really annoying ones are where someone has decided to spam the thread after 8 years, and the spam gets reported and deleted. The thread is re-launched into active, with nothing new to show.
The only case where it would seem reasonable to resurrect an old thread is if the OP wants to report back, and surely this could be programmed in somehow?