What are IPv6 proxies, and when do I need them?
IPv6 proxies route traffic through IPv6 addresses rather than the older IPv4 ones. The address space is vastly larger, so allocations cost the network very little, which is why they are the cheapest addresses available and why you can have a great many distinct ones at once.
They fit large scale testing, crawling services that already speak IPv6, and anything needing many separate addresses cheaply. They work when the target site, the DNS setup, the hosting and your own software all handle IPv6 properly, and that is four things rather than one.
The check that decides it
IPv6 support is still not universal. Some sites treat IPv6 traffic differently or serve it incomplete content, and many do not answer over it at all. Confirm your scraper, proxy library, TLS settings and DNS resolver handle it cleanly, then confirm your actual targets do, before buying anything.
Worth knowing
Use IPv6 because the job calls for it, not because the pool is bigger. A cheaper address that cannot reach your target costs more than an expensive one that can.
Other questions
These answers describe how the NullVault proxy network works in practice, across residential, ISP, mobile, datacenter and IPv6 pools.