Does it? If you pretend IPv6 doesn't exist, sure, but that's like pretending UDP doesn't exist because all of your applications use TCP, or only logging traffic going to port 80 because you don't have HTTPS yet.

Every firewall I've come across has a default deny rule for incoming IPv6 traffic, giving the firewall the same properties as any IPv4 network. Host firewalls are the same; anything ranging from Windows Firewall to UFW and firewalld have presets to block all traffic except for the applications you've whitelisted. Once you get to huge enterprise routers managing routable IPv4 addresses and IPv6 addresses the situation may become different, but it's still not that much overhead.

The biggest problem with securing IPv6 seems to be ignoring it assuming that makes it disappear. If you configure your firewall to drop all IPv4 traffic not on a whitelist but somehow manage to forget to add the same rule for IPv6, you should re-evaluate your networking knowledge and maybe get up to speed with how the internet has changed since 2015.