A lot of C#'s reputation for not being viable for Linux came from the other direction and a lot of FUD against Mono. There were a lot of great Linux apps that were Linux first and/or Linux only (often using Gtk# as UI framework of choice) like Banshee and Tomboy that also had brief bundling as out-of-the-box Gnome apps in a couple of Linux distros before anti-Mono backlash got them removed.

Also, yeah today Linux support is officially maintained in modern .NET and many corporate environments are quietly using Linux servers and Linux docker containers every day to run their (closed source) projects. Linux support is one of the things that has saved companies money in running .NET, so there's a lot of weird quiet loyalty to it just from a cost cutting standpoint. But you don't hear a lot about that given the closed-source/proprietary nature of those projects. That's why it is sometimes referred to as "dark matter development" from "dark matter developers", a lot of it is out there, a lot of it doesn't get noticed in HN comments and places like that, it's all just quietly chugging along and doesn't seem to impact the overall reputation of the platform.