Concourse CI 5 supports CC.XML. This is a proxy for unauthorised access to that new API.

concourse-proxy.js

#!/usr/bin/env node
// Concourse CI 5 supports CC.XML
// This small program will allow unauthorised access to the new API, while we
// figure out how to get existing monitors on to OAuth and friends.

const concourseUrl = 'http://localhost:8080';
const port = 8081;
const flyTarget = 'proxy';
const flyExecutablePath = 'fly';

const niceRequire = (module) => {
    try {
        return require(module);
    } catch(e) {
        console.error("Please run 'npm i http-proxy yaml' first");
        process.exit(1);
    }
};

const http = niceRequire('http'),
      httpProxy = niceRequire('http-proxy'),
      fs = niceRequire('fs'),
      os = niceRequire('os'),
      path = niceRequire('path'),
      yaml = niceRequire('yaml');

const readToken = () => {
    const flyrcPath = path.join(os.homedir(), '.flyrc');
    if (!fs.existsSync(flyrcPath)) {
        throw new Error(`Try logging in via '${flyExecutablePath} login -t ${flyTarget} --concourse-url="${concourseUrl}"' first`);
    }
    const flyrc = fs.readFileSync(flyrcPath, 'utf8');
    const config = yaml.parse(flyrc);

    if (!config.targets || !config.targets[flyTarget]) {
        throw new Error(`Try logging in via '${flyExecutablePath} login -t ${flyTarget} --concourse-url="${concourseUrl}"' first`);
    }

    return config.targets[flyTarget].token.value;
};

const isRequestForCcXml = url => /\/api\/v1\/teams\/[^/]+\/cc.xml/.test(url);

const proxy = httpProxy.createProxyServer({changeOrigin: true});
proxy.on('proxyReq', function(proxyReq, req, res, options) {
    if (isRequestForCcXml(req.url)) {
        try {
            const token = readToken();
            proxyReq.setHeader('Authorization', 'Bearer ' + token);
        } catch (e) {
            console.error(e);
        }
    }
});

// fail early
readToken();

console.log(`All ${concourseUrl}/*/cc.xml access points are now available without authorization. Try:`);
console.log(`curl -v "http://localhost:${port}/api/v1/teams/main/cc.xml"`);

http.createServer(function(req, res) {
    proxy.web(req, res, {
        target: concourseUrl
    }, e => {
        res.writeHead(500, { 'Content-Type': 'text/plain' });
        res.write(e.toString());
        res.end();
    });
}).listen(port);

Or use https://gist.github.com/spinningarrow/8ddd202e7e27568c754e71bb53a72b65