Great question, after a bit of source diving the answer certainly surprised me!
A quick look at the Android sources seems to provide the answer. Let’s start by looking back in Android 2.2 at ActivityManagerService.java. Notice around line 186 a constant defined called ACTIVITY_INACTIVE_RESET_TIME that happens to be set to 30 minutes.
// How long until we reset a task when the user returns to it. Currently
// 30 minutes.
static final long ACTIVITY_INACTIVE_RESET_TIME = 1000*60*30;
Look a little further for the resetTaskIfNeededLocked() method around line 7021 and you will see this value checked to determine if the task should be reset before being launched.
Fast-forward to the Android 4.3 sources and the code has been moved into ActivityStack.java that is called from ActivityManagerService, but the basic structure is the same. This time, the constant is defined around line 125:
// How long until we reset a task when the user returns to it. Currently
// disabled.
static final long ACTIVITY_INACTIVE_RESET_TIME = 0;
The same resetTaskIfNeededLocked() method is found around line 1973, and you can see that now it checks if the value is greater than zero before applying the same timeout check to clearing the task state. Notice, though, that this method does still check FLAG_ALWAYS_RETAIN_TASK_STATE, so this flag can still be used to protect a state clear, but it seems that with the outer check disabled this code will never be executed.
Overall, this seems like pretty compelling evidence that the feature has been effectively disabled in AOSP for later versions of Android. I do not see an external means (via system properties, etc.) for this value to be re-enabled per device unless the manufacturer were to rebuild the code with a value added here…but that is uncommon. Most ODMs stick to config properties in XML or system properties that they can control via an overlay.
So while technically the feature hasn’t been “removed”, it would seem to me that the documentation is no longer correct in terms of it auto-triggering after a delay.