It's a fork with a lot of modifications (mostly removing deprecated stuff and making it cross-platform). You can still see a lot of ancient stuff in the sources such as referring to the base Object class as "COM+ object" (.NET was originally envisioned as a successor to COM).
>An early name for the .NET platform, back when it was envisioned as a successor to the COM platform (hence, "COM+"). Used in various places in the CLR infrastructure, most prominently as a common prefix for the names of internal configuration settings. Note that this is different from the product that eventually ended up being named COM+.
Correct, the bytecode wasn't even 1:1 compatible. They then brought over missing pieces, and consolidated .NET Framework features into .NET Core, thus becoming just .NET to end the dumb naming war, since everyone calls it .NET anyway...
> It was forked out of .NET 4.x and dropped support for a lot of things in the first versions.