IPv4 has a lot of reserved and special-purpose ranges. Hell, I don't think most people could even recite the three major private-use blocks. Multicast and link-local addresses have more complications.
IPv6 is similar, though a little better consolidated. While there is one range of addresses reserved specifically for organization subnets at your disposal (fc00::/7), a lot of people assume just about anything not currently assigned for the internet (presently 2000::/3) will be safe forever.
I understand your point, but I think the context is more of "They can't even handle a fizzbuzz level problem" and not come up with something quite naive (i.e. something like naive RegEx matching), rather than "they can't implement a professional IPv4 validator that takes into account all the intricate special cases with subnets and reserved address blocks."
IPv6 is similar, though a little better consolidated. While there is one range of addresses reserved specifically for organization subnets at your disposal (fc00::/7), a lot of people assume just about anything not currently assigned for the internet (presently 2000::/3) will be safe forever.