So, exactly the same as ipv6 then? You don't need to be a net op to understand why ipv6 is better.
To the security issues of ipv6 with respect to firewalls. A: firewalls are a feature that developers should enabled by default at this point and B: If you are silly enough to not use a firewall on all of your devices, let alone something that is meant to be on the wider internet you pretty much deserve the IT Darwin Award/Russian hackers on steroids.
Then forgive me for being blunt, get one. You are having people pay for unraid, an OS primarily developed/used for network accessible storage and you do not have a networking specialist? Why?
I understand you need to prioritise other aspects of the OS (particularly encryption - god knows I want that myself), but at the same time it sounds like you are working with little to no resources (human or otherwise) which is making jobs that might take a day or a week for a team to develop (add on the testing) take 10x that.
Your demand for specific examples where apps won't work due to a lack of ipv6 is banal. It's like asking for examples where "the car will only work if we upgrade its tires." A needlessly specific example? Say I have an ipv6 VPN provider who to save costs no longer has, buys or supports ipv4 addresses. You know as well as I do as does everyone else that ipv6 is a general update none specific to any one app or service.
Ultimately, my argument is about a feature that isn't 100% necessary in the same way premium gasoline isn't 100% necessarry, but it has existed for long time and I think my car should support using it. It's basically the same fuel but I can go faster, though I could accidentally explode my engine however...
Your argument is, retrofitting the engine to support both will take too long (and you need to work on the alarm system anyway) and besides, until more people use the premium fuel, its just for enthusiasts. Problem is, eventually all you will have to burn is premium gasoline.
You need to prioritise other things sure, I'm just salty because you shouldn't be in a position where getting ipv6 working is difficult or taking a lot of time.
It should be given Slackware already has it, just integrate and blacklist the module or disable it via some of the other methods.
The following is semi on topic wrt ipv6...
Depends on who you think the big players are and what exactly your concerns are? State actors don't need IPV6 or IPV4 static IPs to profile individual machines behind a gateway/in a LAN. If its your general skiddie or "Russian" hacker on steroids the effect of a gateway is still fairly minimal. Gateway or not, you run a vulnerable service or device and I'll just pivot from that machine into your LAN anyway. Knowing a ipv6 machine is running a certain service or has a certain port open isn't greatly helpful compared to *some device* behind ipv4 NAT.
On top of that, the general execution of infosecurity via web browsers is so weak you can easily uniquely track any individual's session across the internet irrespective via 3rd party cookies, ad networks and the unique characteristics of your browsers. In short, really you're just patching a broken dam with a band aid by avoiding ipv6.
.p.s. NAT-PMP is still garbage anecdotal or not.