Yes, you can write a non-trivial .NET application on Linux. But if you take a non-trivial .NET application that runs on Windows, the odds are low that it can easily be ported to Linux. And there are almost no non-trivial .NET applications that weren't originally written for Windows.
The result is that if you work with .NET, you're going to be pushed towards Windows.
.NET 5 should supersede both Core and Framework IIRC
In the years since, I've encountered many stories that attempted to make use of that portability. All failed.
I've seen the promise of portability with other software stacks, and know how hard it is. I also know that taking software that was not written to be portable, and then making it portable, is massively harder than writing it to be portable from scratch.
So, based on both the history of .NET and general knowledge, I won't believe that .NET will actually wind up being portable until I hear stories in the wild of people porting non-trivial projects to Linux with success. And I will discount all announcements that suggest the contrary.
Microsoft didn’t have anything to do with Mono until 2016.
If you start from scratch with a greenfield .Net Core project there aren’t really any issues getting it to work cross platform.
While I’ll agree that anything that uses any of the advanced features of Entity Framework and MVC is not trivially ported.
So yes, even in a new project, there will be a pull back to Windows. Because virtually nothing is truly written from scratch.
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?