ZX Spectrum FTW!
Yes, and this theoretically leaves a tiny chance for supporting a 16-bit CPU, or at least pretend to. There’s an interesting beast, Motorola 68012, this CPU (as well as 68000 and 68010) is often called 16/32-bit processor, because it uses 32-bit registers, but has a 16-bit external data bus. What’s more interesting for us is the CPU uses 31-bit address bus, allowing it to address 2GB of flat address space. That doesn’t mean there ever was a computer with 68012 CPU and RAM amount enough to run Haiku, so even if 68012 port is implemented Haiku could only run on a 68012 emulator.
But that’s just about dealing with the memory requirements. There’s more than that, like presence of an MMU: Helping on m68k | Haiku Project , or CPU tick counter: Haiku on 486? - #6 by VoloDroid . So no, porting Haiku to a 16-bit CPU wouldn’t be an easy task at all.