You can “dual license” your code, proprietary and whatever strong copyleft you prefer. For users, it means copyleft, and for steam it means your code is compatible.

Nightshade doesnt actually work btw. Denoising, a common technique, also breaks nightshade completely. Its also closed source, with no way to test if it actually works for the big AIs. The person making nightshade is really fishy too.