There are more and more IPv6 only machines in the internet. I also manage some of them and was regularly hit by missing IPv6 support when building ports.

I did some research about the impact of missing IPv6 support on the ports-tree. The result is, that 10.308 of 25.522 ports are not fetchable when using IPv6. This renders - through dependencies - a total of 17.715 ports unbuildable when having only IPv6 available. All you can do than is waiting and hope, that distcache.freebsd.org caches the distfile. But this will take some time, which you maybe not have when hit by an security issue

Based on the research a promotion campaign for IPv6 was started. Some volunteers will contact the related administrators and try to convince them to support IPv6. This will start in January 2016 and will hopefully create some progress soon.