I wouldn't call it the standard, it's just used in designs with legacy to avoid the huge cost of re-qualification of hardware and software. It's infeasible a lot of times due to cost and power consumption. I work in the private sector in space (lunar exploration actually) and everyone is qualifying normal/automotive grade stuff, or using space-grade microcontrollers for in-house designs, with everything from 8-32bit, [1] and ready-made cpu boards[2] for more complex use cases. I'm sharing just 2 examples but there are hundreds, with variations on redundancy implemented in all kinds of ways too, such as in software, on multiple cores, on multiple chips, or on multiple soft-cpu cores on a single or multiple FPGAs.
Reliability is more important. Even more problematic is that many semi companies have been funneled into just a few due decades of mergers. And all of these are chasing profits which means jettisoning RAD hard mil spec devices. Up until the early 2000s intel was still making hardened versions of the 386, now they make no milspec parts.
[1] Example: https://www.militaryaerospace.com/computers/article/16726923...
[2] Example: https://xiphos.com/product-details/q8