Great question, @GerryH!
My impression is that the automotive industry is one of biggest customers for the Qt Company (TQtC) at the moment. 3D graphics and Qt Quick are vital to the automotive IVIs, so TQtC and their partners (like KDAB, a major contributor to Qt 3D) will continue to pour lots of resources into them. In general, I believe that any effort you invest into Qt Quick and Qt 3D is quite safe for the next 5-10 years.
The path forward isn't easy -- Vulkan, Metal, and DirectX 12 support are all far far behind the level of OpenGL support, so it will be years before they catch up. Nonetheless, my gut feeling tells me that dropping support for Apple is unthinkable, so TQtC will adapt to Apple's demands. And given Apple's history of viciously dropping backwards compatibility, the sense of urgency is definitely there.
Some possible ways forward are:Use MoltenGL to keep the OpenGL backend usable on Apple platforms (TQtC either forms a direct partnership with Molten, or Qt customers buy MoltenGL themselves), OR Add abstraction layers to decouple Qt from OpenGL and add a Metal backend directly, OR Like #2 but provide a Vulkan backend for Qt 3D and Qt Quick, and then use MoltenVK to support Apple platforms (like how OpenGL + ANGLE is currently used to support Windows)
Personally, I think #3 makes most sense because Vulkan support directly benefits Qt on all platforms, not just Apple platforms. However, I can't tell if this is a feasible business strategy for TQtC or not, or what kind of tradeoffs exist compared to a direct Metal backend.
But anyway, for a more reliable source, this recent email from a TQtC employee suggests that they are currently planning the direct-Metal route: https://lists.qt-project.org/pipermail/interest/2018-December/063503.html There should be a more formal announcement sometime this year.
(Disclaimer: I, too, am a mere community member who lacks a crystal ball)