A stateful firewall is prerequisite for NAT implementations commonly deployed in most office and consumer settings due to the session tracking requirement. So you just stop doing the NAT part and the firewall continues to deny untracked ingress connections just like it did when NAT was running.
NAT is only needed if you want to transition from a private network to a public one.
ipv6 still needs nat configuring. nothing changes there.
The only thing that changes from a network administrator perspective is it becomes much harder to ensure devices that should only have a private ip address do not have a public one.
