If a server has no AAAA record, machines only connected to IPV6 simply cannot contact that server. This is fine in theory (just buy an ipv4 lolz) but in practices this means additional costs that will only go up, as the ipv4 space becomes more and more crowded. As a non-network admin, I care that services I'm a business costumer of are available to my servers, the biggest offender being GitHub. GitHub still does not have an ipv6 despite "looking into the issue" for several years, and this means that I am unable to use their services or indeed any software that requires access to GitHub for, say, updates, pulling artifacts, container images or whatever else from their servers.
It does sound like it's a bit presumptuous of me as a random IT person to demand an IPV6 out of them, but in the year of the Linux desktop, there's no reason for them not to have the infrastructure in place to simply flip a switch and be done with it, for basically free.
Of course, the big players own swathes of IPV4 space that they are able to rent out at a premium, so they are heavily disincentivized from ever supporting ipv6, as that means obsoleting their investment with no replacement, but it's bullshit that they are able to do this in the first place.