I do not see any rules about whether or not to reply to an old thread, but it seems that these days “thread resurrection” is generally frowned upon in web forums.
In my case, the response to a thread from a year ago was the top hit on a web search for my problem, and it quickly solved my problem. I instinctively started to respond to the thread saying thanks and indicating how my situation was similar. But the forum interface asked me if I wanted to reply to a year-old thread, leading me to believe it may not be a good idea.
The “keep things tidy” rule in the FAQ could be interpreted as suggesting avoiding thread resurrection, but that’s the only thing I saw. I could DM the person to say thanks, but DMing random strangers in a case like this is kind of awkward.
Is there an official stance on this situation? (And if so and that’s clearly posted and I’ve missed it, my apologies.)