That's a feature of the new forum software and something that I and, from comments in the mod forum, the other moderators are doing. Same with individual posts - the "delete" dialog now has a field where we can send a message to the person who posted the thread or the individual post, respectively. I believe the message goes to the OP as a PM. If you're not ever looking at your PM box, you might miss it....
It would be nice if OPs were informed, by default, of a threads deletion though.
If any of you would like a test/demo, post after after my post here with the text, "Please delete this." I'll check in a little later and do so, with a message to you about it through the system. Let's give the system a test. If it's not working for some reason, we'd like to know about it.