Although stateful connection tracking is still a best practice for IPv6 firewalls and home routers.

My firewall doesn't do NAT yet is still stateful for IPv6.

Also, most firewalls on end-point devices (think built-in to the OS) are stateful firewalls.

There are a ton of IPv4 firewalls and middleboxes that have stateful tracking even when NAT is not in use.