It won't be something you as a layperson can just take and run in your car, like you would Linux on an old laptop. The HW is pretty diverse and is tightly coupled with the car. It's more like something car makers can use to build on top of in order to quickly and safely bring up an ECU. People who worked with, for example, AUTOSAR stacks on modern ECU's know how much of a pain bringing up even just CAN communication can be.
I share your concern about bitrot and longevity in modern cars, and this could help, but it would still not be something someone could just do in their garage, you'd likely need more resources than that.
There are 2 ASIL-D OSEK implementations off the top of my head, Tasking, the EB one ( I think they actually have _2_ variants here, the normal one and a microkernel) and I'm sure there are others, these being just the ones I saw on projects I worked on before Oxidos.
I'm sure Vector has one as well.
Edit: You are right, that was poor form, I added the disclaimer.
Regular cars have a lot of OS'es in them that are not QNX. I'd say OSEK derived OS'es are much more common than QNX. And I believe there is quite a bit of space for alternatives.
But they weren't on my list because those have ARM cores and the support for those in Rust is quite good.