Can you give me a .NET assembly (DLL), that I then load, run in a completely safe sandbox with practically zero risk, and then unload?

Is .NET sandboxing effective yet? It's my belief that if I want to do something like this, that I'm way better off using V8 or some other sandbox to host your code. Or use Deno or something.

Or is it still not possible to run someone else's .NET code really safely?

Asking for a friend who wants to make an online game in the style of C-ROBOTS, but is afraid of running other people's code.

In theory, they could create a wasm binary I know they are starting to incorporate more of it via blazor among others

Wasmtime includes support for .Net:

https://github.com/bytecodealliance/wasmtime/