This is actually something Tech brought over from Mumsnet - we share the same tech team, and often when one site has an update, the other does too. It's to alert users if they're responding to a thread that's dormant - it doesn't mean you can't respond to an old thread of course.
I personally don't like it and see no reason for it to be there,there are many early threads which can be revisited and read when new members join and have a search through the Forums so every reason to leave them where they are.
Actually, phoenix, I started a thread about the Zombie Thread notice waaaay before you! I wasn't going to say anything, but can't let you get away with that!
I received such a shock today and thought that I had a virus! I was not aware that I had posted anything on an old thread. I have never seen anything like that in my life and hope that I never do!
I have never seen this before and was initially alarmed. I take the point that some threads go back a long way and may have become dormant often with good reason. I often wonder about why they are revived, but have mistaken dates myself and found that I am replying to something from 2012 instead of this year! However..........I am not taken with the "Zombie thread alert" It may be that there is something still to add/ask/contribute - OK sometimes not. But could we not have a slightly less alarming way of advising those about to post?