One clarification, since my own language was sloppy. C# did not get open sourced in 2014. That was the date that Microsoft open sourced just about everything (and that's a weasel word there - to my knowledge, it is everything) that wasn't already open source. Things like ADO/ASP/Windows Forms/etc were open sourced back in 2008, along with Microsoft's implementation of their framework libraries, which Mono rapidly integrated into their implementation.
And so on that note, much of that information on the Wiki page is outdated by at least a decade. Mono, especially since their 2.0 release (back in 2008), has been a fully fleshed out, cross platform, production ready alternative to Microsoft's implementation. So I certainly wouldn't call C# a second class citizen on Linux by any means. The one and only reason Linux is not my primary development platform is the lack of Visual Studio.
Ultimately C# started out pretty awful. The language was a mediocre java clone, performance was abysmal, and the language itself was lacking in features. But that changed relatively rapidly, and certainly today it bears little resemblance to where it started. And I think that is perhaps the problem, people seem to think C# of today is the C# of 2004, but I don't understand why that degree of misinformation is so strongly exemplified in this particular language. I think it's particularly a shame because of what a successful tool the language has evolved into.