I personally don't know of a seat sensor recall, but I don't think it would be hard to find, or very expensive for a used seat.
If you swap your Occupant Detection Control Module over to the new seat, and then install the seat, you won't need a dealership to touch it.

If you don't swap your ODCM to the used seat, then it probably will need a seat calibration from the dealership.

 

I think the recall was for the passenger-side front airbag, not the airbag in the seat. Did they look at the electrical connector for the occupancy sensor? Seems like a strange diagnosis.

 

If you are certain this is the same problem as addressed by the recall, then you should speak with Subaru America. Your dealer seems to be saying it's the sensor, not a harness issue (the recall). I'd want them to double check that connection before spending more money.

Vehicle Detail Search - 2015 SUBARU FORESTER SUV AWD | NHTSA

www.nhtsa.gov

Summary

Subaru of America, Inc. (Subaru) is recalling certain 2015-2018 Forester vehicles equipped with heated seats. An electrical connection in the front passenger seat for the Occupant Detection System (ODS) may loosen.

Remedy

Subaru will notify owners, and dealers will inspect and, as necessary, replace the ODS sensor mat harness, free of charge. The recall began February 11, 2020. Owners may contact Subaru customer service at 1-844-373-6614. Subaru's number for this recall is WUM-98.

 

That is good info. I did not know that some models had a recall for the seat occupancy sensor. Might be worth checking the connections under the seat.

 

Original poster I'm dealing with your SAME exact issue as yours (already had 1 seat replacement, not covered under warranty sadly but a known issue for Foresters within this range, and now same error message is resurfacing again). The dealership is trying to point the finger at me saying that I must have wet the seat a second time and retriggered the issue. I'm actually selling my 2015 due to these nightmare issues. A/C has been a lemon too, multiple expensive fixes.

 

Subaru dealership just contacted me today - $1,200 to fix and almost a month long wait. Can't sell this car fast enough. $2,400 in 2 years not covered by warranty to fix is just absurd!!