I think the forum is the only part of the site that is still active. KDAB has their own e-mail newsletter and blog, ICS hasn't posted anything in their part of the forum for years, and even though there have been multiple developer days since 2013, these tend to get advertised directly by the official Qt folks on their site.

I don't subscribe to the RSS streams, but both of those seem like they are still active.

For me, I learned Qt from the tutorials and examples that come with the Qt distribution, by reading many of the Qt books (I found Summerfield and Blanchett especially helpful but they are dated now), and by reading and asking the occasional question on this site. But Qt has become a huge platform and I don't know even a small amount of it now.