I have had the battery disconnected in my 2019 Outback Limited 2.5 with 8" HU (nav) multiple times, and there is really nothing which needs to be backed up. The HU keeps all its settings, as do the seat memories. The only thing you will need to do is retrain the 2 front windows for auto-open/auto-close: drop to halfway, and then close and hold for about 1 second. Note that the driver's side control for the passenger window will not work at all until you retrain the passenger window right at the passenger door.
The only other thing to be aware of is that it will take a few drive cycles until the OBD "readiness monitors" are all set, but that only would affect a state smog inspection.
Also, there is a note in the users manual about waiting 10 seconds before starting the car, since the throttle plate controller needs to run a self-calibration. Haven't noticed this one though. The car runs fine after a battery disconnect as well; I haven't noticed any idling or other driveability problems after each battery disconnect.
So, no need for battery backup.
