This.. is actually what I have going on.
I have unwisely deferred repairing it, but it would be in my best interest to fix it. Some are recommending reupholstering, but considering the actual support of the seat is damaged, a new seat seems like the only true way to fix this.
To put it into perspective, I have a 2014 with the 5-speed manual trasnmission, with over 100,000 miles of various driving, to include plenty of stop-and-go city driving, freeway driving, and harsh/severe driving (high heat, continuous high-RPM use, off-road, etc.). The left side has completely been ruined, and I feel that having the stick shift is the single biggest contributor for the issue on my end.
Seats that are almost certainly compatible include:
2013-2017 Subaru XV Crosstrek; Subaru Crosstrek
2012-2016 Subaru Impreza (non-turbo)
2014-2018 Subaru Forester
2015-2019 [expected 4th gen production model year run]
Subaru WRX and WRX STI