There's seems to be a lot of misinformation on the internet as to when they permanently stopped using the PTU's without the drain plug and permanently started using PTU's with the drain plug. If there's even exact time frames. I would just drive it up on ramps or put it on a jack stand and crawl under and eyeball it.
Not misinformation. It's just Ford's stupid illogical sense of setting them up. Basically there are several PTU configurations:
1). Has fill plug only
2). Has drain plug only
3). Has fill plug AND drain plug
Usually if there's a drain plug, there's a water-jacketed cooler as well and the fill plug gets deleted in favor of a temp sender.
On the smaller PTUs used on the Fusion Sport and Escape, if you get a cooler, you also get the fill and drain plugs. However, some vehicles like the Edge Sport / ST were reported not to have a cooler.
There wasn't any MY cut-in times or anything like that, it depends on the size of the PTU (model of vehicle) and cooling/non-cooling configuration.
I heard there was a final design change in the idler gears that was cut-in June of 2016 that supposedly fixed the overheating issue (gears were walking into the case, like a Torsen differential) but I haven't heard of any MY17-19 PTUs overheating on the Explorer, which is a good sign.