Similarly to what was discussed in https://forum.qt.io/topic/159077/moving-away-from-qt3d-is-qtquick3d-ready/2 I'd say you might need to evaluate if you can:
Rely on QtRHI for a new rendering ad-hoc system
Check if Qt Quick 3D is enough for your project.
Because it's not tied anymore to the qt meta repo, we might face issues from the Qt for Python side of things, and we might stop providing it in newer versions. So far, it's present in 6.9, and hopefully we can provide it for 6.10.
Like any piece of software, being deprecated since 6.8, Qt3D will not get new features, or might not have active development. I wouldn't expect for it to go completely away in 1-2 years, but it's highly probable that it might be fully replaced in a few years.
This is only my 'feeling' and not what it will happen.