My guess (not a kernel developer): Technically I think it could be built as a dynamically loadable kernel module and shipped separately from the kernel image u

My guess (not a kernel developer): Technically I think it could be built as a dynamically loadable kernel module and shipped separately from the kernel image under /lib/modules. This does not make sense if the functionality is always needed, in which case it should be statically compiled in. In any case there is no benefit of developping separately considering that the module must call functionality from the kernel (at least for the entropy part) so the code is tightly bound to the kernel (and to specific versions of the kernel at that). The functionality could also live in userland, and the kernel could start a process, which links the libsiphash shared library. The process could then communicate with the kernel by other means, reducing dependencies. And of course that would be too slow.