I reached out to my aftermarket support at the dealership regarding this question. He had also heard of the 80/20 rule, but has no official information from Subaru. He recommended it as a best practice at this time.
I updated my car to charge to a maximum 80% on L2/L1 chargers from the Settings menu. I don’t let it dip below 20%, and charge to 80% for daily driving.
Of note, when you set it to charge to 80% and have the charger lock enabled, you must unlock the car using an unlock button on the driver door or remote in order to remove the charger (even after charging is suspended). When I charged to 100% the first time, I did not need to press the unlock button (it released when I used the touch sensor in the door handle).
I plan to charge to 100% infrequently for a long drive that I perform (usually) 2x per month (the reason I needed an extended range battery).