Just as an update - I posted this in the How To: 2016 Explorer Remote Start forum, but I wanted to close the loop on my own thread in case anyone else has a similar inquiry in the future.
The JS7Z-19A361-A module is a bit different from the ones shown on here for 11-16 Explorers. On this December 2020 module, all of the connector ports and the shock sensitivity knob are on one side, so the mounting will be a bit different. But I installed it and the remote start feature works fine with the stock 315 MHz fob (3-keys, no factory remote start). Lock - Unlock - Lock and bam it remote starts!
The rear defrost stays on when I get in the vehicle to turn it on. I don't remember if this was normal for the factory remote start setups. Does anyone know? I have the climate set to Auto. Even with the manual HVAC system, I noticed the A/C compressor kicks on during a cold winter remote start, possibly for the front defog. I guess it makes sense since all of that can be electronically actuated with the exception of the manual temperature knob.
FORScan (or dealer/IDS) is required to enable Remote Start, the climate features, rear defrost, parking lights on when remote starting, heated steering wheel/heated seats (if applicable) etc...
For my 2018 XLT, it was only a 200A with tow package, so I didn't have heated anything. I enabled the rear/front defrost and HVAC system and set it to Auto. I'm curious how this will work with the manual HVAC system during remote start. I can hear the A/C compressor running and can feel the rear defroster.
I got my VSS from Fairway Ford and the manufacture date of the module was only a few weeks ago (December 2020). The labeling on the module's sealed bag said it was a 900 MHz module, but after thinking about it, the module doesn't need or use the antenna. It'd only interface via OBD2. The existing vehicle fobs/antenna wouldn't matter if it was 902 MHz or 315 MHz.
I don't see many people in this same scenario as myself because just doing a quick Autotrader used vehicle search, almost every 2017-2019 Explorer (even XLT) has the touch screen Nav system with Ford Connect, so they'd most likely have the remote start fob already.