I got it working in wsl.
I had to revert wsl to Ubuntu 22 LTS, and then the Scrirra C3 Steamworks plugin worked.
I then compiled my Steamworks Plus extension on Ubuntu 22 (instead of 24), which also worked on wsl Ubuntu 22 (I imagine it will work on Steamdeck).
As you mentioned about Linux, it seems like a backward and forward compatibility issue. What a mess.
I've seen comments about compiling differently, statically linking libs, etc.
Thanks for the model on how to do the linux port, very helpful for Steamworks Plus on linux.