I feel that, at this point in time (we're in 2018, people!), BYOND could certainly use full IPv6 support - the IPv4 address space may not be exhausted yet, but it will be, soon.
BYOND needs to add support ASAP, I'm pretty sure you don't want to turn away (or in this case, refuse to connect to) new users because they only got an IPv6 address available. I know this might be a huge undertaking, but it needs to be done.
I for one am suffering with BYOND due to CGNAT, since my ISP provides me with a fully working IPv6 address to every device in my LAN, but my IPv4 is CGNAT'ed, making me unable to host worlds inside my own computer. I'm pretty sure you folks have thought about it in the past and decided it wasn't worth it then, but right now in 2018, things are looking quite ugly for IPv4. It's only a matter of time.
Sep 22 2018, 4:45 pm