Quick and dirty hack to get a list of all domains hosted on Forge. Run it in your browser console. You still have to manually remove the server name strings.

list-of-domains-hosted-on-forge.js

let domains = Array.from($('.nav-item')[1].children[1].children);
let domainList = domains.map(function (item) {
    return item.textContent.replace(/(\r\n|\n|\r)/gm, "").trim();
});

console.table(domainList);

You can split on space and filter the defaults:

let domains = Array.from($('.nav-item')[1].children[1].children);
let domainList = domains.map(function (item) {
    return item.textContent.replace(/(\r\n|\n|\r)/gm, "").trim().split(' ')[0];
}).filter(domain => domain !== 'default');

console.table(domainList);