[It’s mentioned in the Intellectual Property information for the Online Play Test,](https://twitter.com/nwplayer123/status/1493811797673054211?s=21) and [datamining reveals it’s integrated in Switch Sports’ “agl” graphics library which is used in other first party games.](https://twitter.com/oatmealdome/status/1493827239687094272?s=21) This could mean that future Nintendo games start using it as well (hopefully XC3 lol).
For those unaware, AMD FSR is a spatial upscaler that tries to make a lower resolution look higher resolution. It might be AMD developed but it works on most modern GPUs. Think of it like DLSS, only they both use substantially different methods to achieve their goal.