When you're trying to write new code, you run into a problem and look for a library that solves it. But all of the libraries that you find are Windows First, and it is not until after you're committed to them that you sometimes discover how they are Windows Only.
So yes, even in a new project, there will be a pull back to Windows. Because virtually nothing is truly written from scratch.
It’s no more of a problem with .Net Core than it is with Python modules that have native dependencies, or Node modules with native dependencies.
You’re not going to mistakenly add a non .Net Core Nuget package to a .Net Core project. It won’t even compile.
Of course you can find Windows only nuget packages for Windows only functionality like TopShelf - a package to make creating a Windows Service easy. But even then, I’ve taken the same solution and deployed it to both a Windows server and an AWS Linux based lambda just by configuring the lambda to use a different entry point (lambda handler) than the Windows server.
You can even cross compile a standalone Windows application on Linux and vice versa.
I use a Linux Docker container to build both packages via AWS CodeBuild.
Would you also criticize Python for not being cross platform because there are some Windows only modules?
So yes, even in a new project, there will be a pull back to Windows. Because virtually nothing is truly written from scratch.