The host operating systems for Qt 6.0 they plan to utilize for continuous integration and testing are Windows 10 2004, macOS 10.15, Ubuntu 20.04, CentOS 8.1, SLES 15, and openSUSE 15.1. Those operating systems will see all of The Qt Company’s CI and verification focus.
Qt 6.0 will support development for mobile targets on iOS (13 and 14) and Android (28 build-time, 21 runtime).
The Qt Company is not planning to support Qt 6 on Windows 7 or Windows 8.x along with discontinuing UWP support on Windows 10.
The Qt community may very well maintain support for these and other environments. As usual, there is also a commercial opportunity for customers to purchase additional, extended support for dedicated hosts and targets.