I'm guessing their LTS is whatever Google's released to an extent. Google Chrome is the basis for Electron, it would be a bad move to be over a year behind Chrome, which would lend to poor support from Google Chrome's dev team. At least that's my take on it, I'm not involved with either project. Maybe someone else here has better input over the matter.
They support the last three major releases, and that's it. So right now that's 6.x.y, 5.x.y and 4.x.y. A new major release is released about every 3 months.
To quote the official stance: "If an application has exceptional circumstances and needs to stay on an unsupported series of Electron, developers can silence the end-of-support warning by omitting the final release from the app's package.json" [0]
3.0.0 was released 10 months ago. If the current frequency of releases stays constant any version reaches end of life after about 9 months.