IPv6?

I think the better question is why would we have any need or desire to move to IPv6 for what is essentially a completely stand alone network.  That's what the private IP space is for.
That question was even more true when what became AREDN was started however many years ago....
 

Standalone - as is NOT able to access the Internet from the AREDN 10.x.x.x  network.  Yes, there are people who insist on enabling that capability, but even in a disaster, that capability should be turned off - in my opinion.  Don't believe that, see what happens to your AREDN mesh when someone plugs in a laptop and it decides to download a half gigabyte of Windows updates over a half dozen AREDN hops.  And that is exactly what that disaster cache laptop that has not been connected to the Internet in months is going to do very shortly after you give it a path to the Internet.  In other words, just at the time you need your mesh, it will get choked due to Internet traffic.
I'm completely ignoring the whole encrypted traffic over Part 97 debate - and almost everything on the Internet is encrypted these days.

Note, that I am NOT talking about tunnel connections which normally are using the Internet, but do not give end user devices access to the Internet.
 

IPv4 is also a lot easier for our user base to understand and work with, and has universal support across all devices including the old stuff we tend to hold on to forever ;) IPv6 was also removed in the early days for memory concerns on the nodes.