Looking at the source code <https://elixir.bootlin.com/linux/latest/source/> for the latest (4.16) Linux kernel, I see support for 31 different major processor architectures currently <https://elixir.bootlin.com/linux/latest/source/arch>. However, the next, 4.17 kernel is due to drop the ability to run on 8 of them <http://www.theregister.co.uk/2018/04/03/linux_kernel_4_16_released/>, due to the fact that nobody is using them any more--or at least, nobody with an interest in implementing new kernels <http://lkml.iu.edu/hypermail/linux/kernel/1804.0/00306.html>. Interesting to read Arnd Bergmann’s observations on what these dead-end architectures have in common: In the end, it seems that while the eight architectures are extremely different, they all suffered the same fate: There was one company in charge of an SoC line, a CPU microarchitecture and a software ecosystem, which was more costly than licensing newer off-the-shelf CPU cores from a third party (typically ARM, MIPS, or RISC-V). It seems that all the SoC product lines are still around, but have not used the custom CPU architectures for several years at this point. In contrast, CPU instruction sets that remain popular and have actively maintained kernel ports tend to all be used across multiple licensees.