I have made a workaround which seems to work for the servers I use currently.

I first detect if the client machine has IPv6 enabled on any of its ethernet/wireless adapters.

var allNetworkInterfaces = NetworkInterface.GetAllNetworkInterfaces();
foreach (var networkInterface in allNetworkInterfaces)
{
    // Don't look at network interfaces that are down
    if (networkInterface.OperationalStatus != OperationalStatus.Up)
    {
        continue;
    }

    // Only look at ethernet and wireless interfaces.
    // This likely is not checking VPNs
    if (networkInterface.NetworkInterfaceType != NetworkInterfaceType.Ethernet && networkInterface.NetworkInterfaceType != NetworkInterfaceType.Wireless80211)
    {
        continue;
    }

    var interfaceSupportsIPv6 = networkInterface.Supports(NetworkInterfaceComponent.IPv6);
    if (interfaceSupportsIPv6 == true)
    {
        IPv6Detected = true;
        Logger.Info("IPv6 connection detected");
        Logger.Info($"Id: {networkInterface.Id}");
        Logger.Info($"Name: {networkInterface.Name}");
        Logger.Info($"Description: {networkInterface.Description}");
        break;
    }
}

Then later on when I go to connect to the remote host if IPv6Detected is true I do a DNS lookup and get the IPv4 address and then connect to the address directly. In my specific case of connecting to cPanel servers it appears to work on my main host. This solution will likely fall apart if client is on a VPN, or host is expecting a hostname and not an IP address.

// If IPv6 is detected we convert the host to an IPv4 address and then connect directly to that address.
var realHost = Host;
if (App.CurrentApp.IPv6Detected)
{
    var ipv4Address = Dns.GetHostAddresses(Host).FirstOrDefault(ip => ip.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork)?.ToString() ?? string.Empty;
    if (string.IsNullOrWhiteSpace(ipv4Address) == false)
    {
        Logger.Info($"Connecting to IPv4 address ({ipv4Address}) instead of host name ({Host}).");
        realHost = ipv4Address;				
    }
}

A related change in the library is: https://github.com/sshnet/SSH.NET/pull/1412/files#diff-26dfdbfb05b31914ef01185808106e1f6940985577213f16f79d2c4c05ef2496

Since then the library uses a Socket constructor which uses a dual-mode socket if possible: https://learn.microsoft.com/dotnet/api/system.net.sockets.socket.-ctor?view=net-9.0#system-net-sockets-socket-ctor(system-net-sockets-sockettype-system-net-sockets-protocoltype)

The library does not currently expose a way to customise the transport, such as setting socket.DualMode = false or binding the socket to an ipv4 interface, but there have been requests for it in the past.

I think you could probably simplify the workaround using Socket.OSSupportsIPv6 (and Dns.GetHostAddresses(string, AddressFamily) overload)

Thanks for the extra info. I figured it was socket somewhere deep inside, though I was looking to override the socket handles internal DNS lookup. Didn't know we could do it by setting DualMode. Either way I couldn't find a way to give the library my own SocketFactory or SessionFactory.

I was using Socket.OSSupportsIPv6 returned true when I had disabled IPv6 on wifi but it was still enabled on something else that was not being used. I did keep it as an overall check around both of the above workaround parts though just incase it returns false I can skip the rest of the stuff happening.

I have not checked my workaround on Windows yet, just macOS. But I assume it is fine.