If there's a feasible way to implement this, I'd like to see support for which browsers support the IPv6-specific proxy functions. (Proxy in the PAC/WPAD sense, not related to JavaScript proxy objects.)

Microsoft's documentation on this is here: https://docs.microsoft.com/en-us/windows/win32/winhttp/ipv6-aware-proxy-helper-api-definitions

Testing these may be tricky, because these functions are generally only meaningful in the context of a proxy PAC file. While PAC files are JavaScript, they're expected to be a fairly limited subset of JavaScript, and it's unclear whether these PAC-specific functions are available to JS code generally.