Sites and services often rely on a lot of IP-based filtering and categorization, at the firewall level, to fight abuse and attacks. Routing IPv6 makes that more challenging. Dropping all IPv6 connections makes it easier. So, if you think you can get away with simply not serving requests from IPv6 addresses, it's tempting to do.

Nope. The packets are completely different. An IPv6 packet can only end up on an IPv6-capable host. If the two are going to speak to each other they need some kind of proxy.

Yes, but I figured a proxy is available via ISP when you have ipv6.

It's not a proxy, but the 6to4 protocol that ISPs use to tunnel traffic. ISPs usually have IPv4s available, so they used it in the beginning of the IPv6 transition. However, there were quickly some problems discovered and the whole thing was deprecated after a few years.

See <https://en.wikipedia.org/wiki/6to4> for more information.