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The protocol is simple enough that it is expected that other implementations will appear, especially in environments such as SMTP, IMAP, FTP, RDP where the client's address is an important piece of information for the server and some intermediaries. In fact, several proprietary deployments have already done so on FTP and SMTP servers.
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using System.Net;
using IPNetwork = Microsoft.AspNetCore.HttpOverrides.IPNetwork;

namespace Xperion.BusinessCenter.Extensions;

public static class NetworkExtensions
{
    public static IPNetwork ToIpNetwork(this string cidr)
    {
        try
        {
            var delimiterIndex = cidr.IndexOf('/');
            var ipSubnet = cidr.Substring(0, delimiterIndex);
            var mask = cidr.Substring(delimiterIndex + 1);
            var prefixLength = int.Parse(mask);

            var subnetAddress = IPAddress.Parse(ipSubnet);

            return new IPNetwork(subnetAddress, prefixLength);
        }
        catch
        {
            return new IPNetwork(IPAddress.Parse("127.0.0.1"), 32);
        }
    }
}

services.Configure<ForwardedHeadersOptions>(options =>
    {
        var knownProxyNetworks = configuration.GetValue("KnownProxyNetworks", "").Split(new[]{',',';'}, StringSplitOptions.RemoveEmptyEntries);
        var networkList = knownProxyNetworks.Select(x => x.ToIpNetwork());

        options.ForwardedHeaders = ForwardedHeaders.All;
        options.KnownNetworks.AddRange(networkList);
    });