First of all, thank 3D Slicer developers so much for working on supporting Qt 6.
When will 3D Slicer support Qt 6? Will 3D Slicer support Qt 6 in 2025 ?
3D Slicer is actively being worked on (by @jcfr) to support Qt 6. See the work being tracked at the below GitHub issue. As of today there has been some work to backport some fixes to the PythonQt upstream. Then with an updated PythonQt, we can update CTK and then finally Slicer to support Qt 6. Expect at least a build option that is using Qt6 in the next few weeks.
The general plan is to issue one more Qt5 release (5.10) and then switch the preview builds to Qt6 for testing.
Will 5.10 support both of Qt 5 and Qt 6 ?
To my understanding 5.10 will be Qt 5 only, and then eventually there will be a 5.11 with Qt 6, and then the next stable Qt 6.
There is a possibility the 5.10 tag has a build option to choose building with Qt5 or Qt6, but the default build option and what the final Slicer 5.10 stable release installer will use is Qt5.