Short version (aka TL;DR):
Yes.. you can swap a 2019 Harmon "Gen 3.1" radio with Carplay and Android Auto into a pre-facelift 6th Gen Legacy/Outback.
Do I recommend it? No. I ended up swapping more than I ever thought would be needed, and even the HVAC clock is stuck displaying 24 hour time.

Long version:
My wife recently inherited a 2017 Legacy Premium after a sudden death in the family. The car is in excellent shape, but the lack of Carplay/Android Auto was notable. Due to the emotional attachment to the car as it is, she would not allow me to change the radio for an aftermarket unit, like I did in the other cars that we own. After a little bit of convincing, she would allow me to do an "OEM Upgrade" to a newer 2018/2019 radio, since that would retain some "originality". This ended up being a bigger rabbit hole than I expected, but I am happy to say that I was 99% successful. I wanted to share what I ended up needing to do to make it work.

First off, the stock 2017 Radio was the 7" Fujitsu Ten unit without Navigation, without an Amplifier, and with Telematics. Most of the 2018/2019 radios do have an external amplifier, which I did not want to have to retrofit, but there is an 8" Harmon unit with Navigation, without an Amplifier, and with Telematics. I checked the service manuals for both the 2017 and 2019 models, and I found that most of the wiring was a direct match, and the differences could be fairly easily corrected. The radio is Subaru part number 86471AL67A. I ordered one from LKQ, and it showed up a few days later.

Once it arrived, I noticed that the HVAC controls were completely different.. nearly a full inch shorter. Back to LKQ, and I ordered a 2019 HVAC control panel to match the radio.

I already knew that the 2018/19 had a different steering wheel with completely different button layout. I ordered an appropriate steering wheel from a similar year Impreza, and the a new set of the proper 2019 buttons from parts.subaru.com. I also ordered the Navigation version of the GPS antenna that goes above the cluster.

I started to test fit the radio and HVAC controls, only to find that the HVAC computer is incompatible with the control unit. Nothing would light up beyond the clock with the mismatched units. Again, looking at the wiring diagrams.. I found that the wiring was the same, just that the unit was updated and apparently uses a different signaling protocol between the 2017 and 2018/19 models. I once again went to eBay and found myself an HVAC computer.

Fast forward another week or two later.. I connected the radio, hvac controls, hvac computer, GPS antenna, and steering wheel.. and almost everything was working! The Starlink mute wire needed to move from one connector to another, and while music worked on USB.. Carplay did not! Turns out that the 2018/19 models have a different Aux/USB module, and the older one is not "compatible" with Carplay for some reason.. It might work with Android Auto, but I did not try that. Back to eBay for a compatible Aux/USB module out of a 2019 Impreza.

Just a few moments ago, I swapped the Aux/USB.. Navigation works! Carplay works! Handsfree works! The only remaining issue that I cannot fix is that the time displayed on the HVAC controls is in 24 hour mode. It looks like the 2018/19 cluster has an option to switch that, but the older 2017 cluster does not. The time is correct (syncs with the GPS), but is just permanently in 24 hour time. The time displayed on the radio is correct and in 12h mode, so my wife is okay with it.


Parts used:
Radio - 86471AL67A. (2019 Legacy/Outback w/Navigation and w/o Harmon-Kardon sound)
HVAC Controls - 72311AL32A
HVAC Computer - 72343AL53A (72343AL52A would also work)
GPS Antenna - 86277AL13A
Steering Wheel Buttons - 83154AL85A
Aux/USB Module - 86257AL13A
Steering Wheel - From 2018 Impreza. Any Subaru wheel of the proper style will do the job. Finding one with the horn pad is rather difficult. I ended up getting mine from Japan.


Photos
Old Radio and HVAC:
Name:  20210707_141414459_iOS.jpg
Views: 40
Size:  142.5 KB

Old HVAC computer vs New. Old has black label, new has blue:
Name:  20210707_151718682_iOS.jpg
Views: 41
Size:  98.2 KB

New style steering wheel installed:
Name:  20210707_155702072_iOS.jpg
Views: 41
Size:  120.9 KB

Final product, Carplay showing with new Impreza Aux/USB Module installed:
Name:  20210714_142601677_iOS.jpg
Views: 40
Size:  127.0 KB