In practice the actual available usable address space for userland is 64 TiB due to user/kernel split and the kernel maintaining a virtual mapping of the entire physical address space (minus I/O ranges) [0].
However newer incoming 5-level page intel chips [1] will allow up to 57 bits of address space, 128 PiB in theory though in practice 32 PiB of userland memory. See also [0] for discussion on practical limit for 5-page too!
True, though /proc/cpuinfo only reports the size, which is ultimately what the CPU cares about. Plus the most relevant limit is what your motherboard and wallet supports, which is often far lower.
Indeed, and as you say, sensibly speaking you are hardly likely to hit those limits in any likely (esp. home) setup. The actual meaningful limit is usually the CPU physical one as home CPUs very often have stringent memory limits (often 32 GiB or so) and of course you rely on the motherboard's limitations also.
Having said that I did write a patch to ensure that the system would boot correctly with 256 TiB of RAM [0] so perhaps I am not always a realist... or dream of the day I can own that system ;)