Another reason against IPv6/the reason I believe many large social media platforms, payment processors (e.g stripe/paypal) and more don't support IPv6 is the huge decrease in cost of IPv6 proxies vs IPv4 proxies.
A proxy, or more in these terms a "residential proxy" is pretty much a VPN but you get someone else's real IP, so to any website or service you look like your a real user connecting without a VPN.
People sell proxies through mobile sim farms, and some platforms run a "sell your bandwidth" service, and route users traffic through a pool of users computers (obviously a bad idea to be on the exit-node end of this)
The best (and really, the only legal way/not against tos) to resell an IP your owned/assigned is to lease the IP directly from a provider as a business.
Some of the biggest proxy providers lease IP's directly from USA's AT&T, T-Mobile, Sprint and RCN, UK's O2 and host them in a datacenter, where they are re-sold to users at about $1.50/IP/Month. This is still cheap, but, with IPv6 you can purchase IPs at just $0.06/IP/Week (If your buying a substantial amount of IPs you can get it at around $0.03), which makes it much cheaper to run a bot farm on any website that supports IPv6.