Why would you think it's rare? I've seen places running multiple projects were developers were barely aware that the sysadmins gave everyone an IPv6 address, an

Why would you think it's rare? I've seen places running multiple projects were developers were barely aware that the sysadmins gave everyone an IPv6 address, and therefore the rate limiting has to be IPv6-aware. It's trivial to fix once it becomes a problem (at least in our case it was), but I wouldn't expect it to be an uncommon mistake.