From the documentation:

worker.suicide:

Set by calling .kill() or .disconnect(), until then it is undefined.

worker.disconnect():

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

Causes .suicide to be set.

Repro:

var cluster = require('cluster');

if (cluster.isMaster) {
  // Master forks and listens for events
  var worker = cluster.fork();
  cluster.on('disconnect', function(){
    console.log("disconnect", worker.suicide);
  });
  cluster.on('exit', function(){
    console.log("exit", worker.suicide);
  });
} else {
  // Worker just disconnects
  cluster.worker.disconnect();
}

In node v0.10, both log statements would print true for worker.suicide. In 41b75ca, which landed during the v0.11 branch, this was broken, and all versions up to node v4 will print false. This is because worker.disconnect(), when called from the worker, does not send the suicide message anymore -- only worker.destroy() will send that message to the master.