It seems like this abandoned feature of three.js has become somewhat of a coveted prize; I've attempted to incorporate it a few times without success.
You can find an example that works on an older release here:
My minimal broken example is available here: https://jsfiddle.net/bitsofcoad/1k3arL33/
var width = 20;
var height = 20;
var rectLight = new THREE.RectAreaLight(0xffffff, 500, width, height);
rectLight.intensity = 500.0;
rectLight.position.set(0, 30, 0);
rectLight.lookAt(mesh2.position);
scene.add(rectLight)
var rectLightHelper = new THREE.RectAreaLightHelper(rectLight);
scene.add(rectLightHelper);
I have ensured that the proper rectarealight library is included in the jsfiddle.
This thread was very informative about why this PR faced some challenges: https://github.com/mrdoob/three.js/pull/9234
And this thread was fascinating to see how others had progressed in developing a similar light type for three.js:
Improved Area Lighting in WebGL & ThreeJS
I'm not sure what caused abelnation's example to break, or if it is entirely broken (I may have messed up my jsfiddle...), but I believe there is still significant interest in this feature.
The soft edges look so appealing, I'm tempted to borrow WestLangley's custom shader (http://jsfiddle.net/hh74z2ft/89/), which pretty much has all the functionality I require.
Has anyone else managed to get the basic functions working? Thank you!