Conclusion 

Can be used as a static IP. 

Explanation 

The IP assigned when creating the NLB or when adding a subnet in an Availability Zone will continue to be used until the NLB is deleted.

To give a bit more background, CLB and ALB use a node-based architecture where each node has its own network interface and IP address. In this case (node-based), if a node is changed or added, the IP may also change or be added.

However, NLB is not based on individual nodes but is created in a form similar to a clustered appliance, with fixed network interfaces and IPs. After creation, only interfaces can be added.

In fact, on the NLB console, you can see the following message on the screen where you add Availability Zones/subnets:

"Choose your Availability Zones and subnets with care. After you create the load balancer, you cannot disable the enabled subnets, but you can enable additional ones."

-> In short, once created, only additions are allowed.

In summary, unless you add Availability Zones/subnets after creating the NLB, the IP set at the time of initial creation will not change.