I have no problem switching Linux to Qt6 (assuming we can fix at least the symmetry plugin which had some linking problems).
To a large degree, I suspect package maintainers will switch to Qt6 anyway - and they support a large part of the user base.
I’m worried about releasing a 1.100 with Qt6 for Windows since there’s clearly a launch bug with some fraction of Win10 users … and it’s hard to get anyone experiencing it to test nightly builds. I don’t blame them – who wants to run a program that crashes on startup?
So I want a lot of time to test Qt6 deployment on Windows (e.g., the OpenSSL bits will need testing / updating there)
Qt6 on Mac is do-able, but it’s a slog because all the libraries, plugins, etc. need to be properly run through install_name_tool to make sure they look with relative paths for each other. The default macdeployqt gets most of the way, but nailing down the pieces will take time. (Obviously the big bonus is being able to ship an ARM / x86-64 universal binary.)
So if you’re asking a release plan - it would be to have a 1.100.1 or a 1.101 maybe February or March. I wouldn’t be surprised if there are a pile of other minor bug reports that crop up. That gives time to test Qt6 on Mac and Windows.