The Slack desktop app was originally written us the MacGap framework, which used Apple’s WebView to host web content inside of a native app frame. As this approach continued to present product limitations, Slack decided to migrate the desktop app to Electron. Electron is a platform that combines the rendering engine from Chromium and the Node.js runtime and module system. The desktop app is written as a modern ES6 + async/await React application.
For the desktop app, Slack takes a hybrid approach, wherein some of the assets ship as part of the app, but most of their assets and code are loaded remotely.