Back in the day there were no protections. You could write to any address whether it was used by the CPU for interrupt vectors, part of the OS, hardware addresses, anything.
Sure, but I'm asking if that's something that's enabled by default today or not. I don't see why it's unreasonable for things that were useful "back in the day" to not be available with the default arguments on current versions of compilers but available with certain flags. I'm not sure why my question touched a nerve, because I'm genuinely asking both if I understand correctly and if it's something that needs a flag to enable.
In older CPUs there was no memory management hardware and no virtual memory. You could just read/write in code from any address anywhere and you'd be writing to that actual physical memory in the computer. This wasn't a feature so much as it is a lack of a feature.
Modern CPUs with virtual memory means the question is a lot more complicated. Every process in a modern OS gets it's own address space so you can write to 0 but it could go anywhere (even virtualized to disk) and all the actual hardware is not directly accessible (must go through the OS).
I'm not sure I'd call this ability "useful" except if you're writing an operating system. This is vast simplification but when your computer boots it's effectively in a mode that allows reading/writing to anywhere. The OS kernel has direct access to all the hardware and then it limits access when running user processes.