What's the point? There is no public way to add non enumerable property anyway

Yes, this is indeed very edge case but we are using Object.keys() for regular properties not Object.getOwnPropertyNames() so one reason is consistency.

This makes it a tiny bit slower for no additional benefit. There is no public way to add a non enumerable property to _events and we don't do it internally.

If anything, it could be nice to use Object.getOwnPropertyNames instead of Object.keys since that guarantees the return order (although to be fair, keys is well ordered in practice too).

What about some madness like this:

'use strict';

const EE = require('events');
const s = Symbol();

Object.defineProperty(Object.prototype, s, {
  get: function () {
    Object.defineProperty(this, s, { value: 'foo' });
  }
});

const ee = new EE();

ee.listeners(s);

ee._events.hasOwnProperty(s); // => `true`
ee._events.propertyIsEnumerable(s); // => `false`

Ofc if you do that you are crazy but that's an example of how a non-enumerable property can go into _events using public APIs.

If someone definePropertys on Object.prototype a lot of code in Node breaks anyway since we have plenty of places where we use objects as maps. This would be the least of our worries most likely.

Ok well these were the only reasons behind this change, I close this.