I tried to install Windows 7 under Qubes R4.2 by restoring a backup of a Windows 7 template (without Qubes Windows Tools, aka QWT) from Qubes R4.1. This was possible without any problems.
In this system, QWT 188.8.131.52 could be installed, including the PV network and disk drivers, also without any problem. The resulting system supported seamless mode, clipboard and file copy to/from other VMs. So it looks as if Windows support is working under Qubes R4.2, too.
I’ll check with Windows 10 and 11 in the next days.
I think I will soon try a Windows 10 VM. (I don’t care too much about seamless windows–in fact I find that they often go inexplicably transparent, so I essentially switch out of seamless as soon as I get the Windows 7 VM started.)