Currently, many public web sites that allow access via IPv6 do so via proxies. This is seen as the “quick fix”, as it requires minimum changes to the site itself. As far as the web application is concerned, all incoming traffic is IPv4.  The most obvious issue here is logging, in that the application only “sees” the proxies IP address, unless it inspects headers added by the proxy, which will no point to (unreadable?) IPv6 addresses.