A couple of years ago, I made a blog post, KDE makes Qt, with data about which percentage of Qt contributions came from people starting in KDE. Basically, how many Qt contributions are made by people who used KDE as a “gateway” drug into it.
I have now updated the graphs with data until the end of September 2017:
Many of these changes are made by people not directly as a result of their KDE work, but as a result of their paid work. But this doesn’t change the fact that KDE is an important project for attracting contributors to Qt, and a very good place to find experienced Qt developers.