I might be missing something here, but does this mean that .NET Framework and .NET Core have diverged, and you need to take extra steps to keep code compatible with both?

The major differences between .NET Core and the .NET Framework: [...] APIs — .NET Core contains many of the same, but fewer, APIs as the .NET Framework, and with a different factoring (assembly names are different; type shape differs in key cases). These differences currently typically require changes to port source to .NET Core.

While I understand the motivation, this at first sight looks like something that will be with us for a long time, and could make life more difficult especially for library authors, who need to potentially target both 'platforms'.

[Disclaimer: haven't used .NET technologies for a very long time and might be horribly wrong here]