It's available but not in common use, and where it is used it is used mostly as a portability option (though it is very well possible that is no longer the dominant use, I just haven't seen it used any other way outside of the Microsoft eco-system).
Main deployment platform for back-end workloads using .NET is Linux. There are GUI frameworks which support it (Avalonia and Uno), file and sockets I/O support is first-class. CLI tooling is platform-agnostic, paid and free tools for developing in C# are available under Linux and macOS. With some effort it can be also run on FreeBSD (not mentioning interesting but niche projects like BFlat which can target UEFI directly).
For example, getting the SDK on Fedora is just
sudo dnf install dotnet-sdk-8.0
It is as cross-platform as it gets, far more easier to manage than installing JVM implementation and then dealing with a switcher or just the fact that Java needs Gradle to build projects over trivial dotnet new console; dotnet run/build/publish
In my experience, the only places pushing for C#/.NET in Linux are places that were already C#/.NET before the core started to support it.
It's still fairly popular but that popularity is waning as languages with more modern design principles are gaining momentum without relying on a heavy framework like .NET. This could make it a poorer choice for somebody just learning as there may be fewer opportunities for junior devs by the time they graduate.
Even Microsoft has started transitioning to Rust in some cases. I'd hesitate to recommend a language to somebody getting started if that language is under publicized risk of being replaced by its maintainer.
I would add that a purely OOP language also probably isn't ideal for a first language, though. Being able to start teaching with just functions is quicker to introduce than having to describe objects first.
An OOP centric language is definitely appropriate for a second language. But if we are talking "intro", the faster somebody can type hello world while still having more programming concepts in the file than just the print statement, the better. For that last statement, I'd also exclude Python.
I always recommend talking about goals first then recommending a path.
If they know web dev is their future, JS.
If they know IOT is their future, C.
If they know games, probably still C as C++ is still likely the path for a while.
We are talking first language. Not people who are looking for a job right now. What about that doesn't make sense? These are people who are likely months to years away from getting a job.
