I don't recall them mentioning any updates being planned for release in September, though (and I could be wrong) I think they said they're aiming to do a small update (fishing update?) before Mistlands. My personal estimate for the Mistlands update (back in February) was that it would release in July at the earliest and October at the latest, but I don't think the developers have actually specified dates for ANY content updates since they released the Ice Cave update.

I'm still thinking we might see an October release for Mistlands because it would make a lot of sense thematically (spooky area, Halloween, roughly a year since H&H, etc) buuuut... the developers have made some posts that indicate there have been some issues/choices that have set the release date back a bit so I wouldn't get your hopes up for any content updates before the end of the year.