Why is Shotcut using Qt 5.6.1 and not the latest from 5.6.X series, which as far as I know today is Qt 5.6.3?
It is not easy to make new custom builds of Qt with QtWebKit for Windows especially the 32-bit. I am working on a Qt 5.10.x actually under a new process that is easier to manage for updates. It is not done and was de-prioritized for more critical fixes. macOS and Linux are much easier to upgrade Qt, but I want to keep my builds consistent across platforms (Flatpak is an exception since it was contributed and how Flathub works).
I understand the choice of Qt 5.6, since it is an LTS version, I just would expect you to use the latest release of the 5.6, which is 5.6.3 AFAIK.
Considering the Qt 5.10 version, it would be supported until 30.10.2018. On the other hand, using the latest LTS version, which is Qt 5.9 (latest release AFAIK: 5.9.6) sounds better since it would be supported until 31.05.2020.
As a source for this information I used: https://en.wikipedia.org/wiki/Qt_version_history
However, I understand the challenges you face to make new custom builds of Qt with QtWebKit for Windows.
I meant 5.9.x instead of 5.10.x