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.
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.
Development is actively in progress. See recent work prepping for Qt6. It will be a build option first. Slicer Stable release 5.10.0 is expected to be released this week using Qt5 and Slicer Preview will switch to Qt6 likely soon after that.