The existing codebase are not directly compatible because .Net Standard 2.0 require at least .Net Framework 4.6.1 or .Net core 2.0
Advantages of using .Net Standard 2.0 is native compatibility with Windows, Linux, MacOS... (And more?). Also .Net Core is faster then .Net Framework
https://blogs.msdn.microsoft.com/dotnet/2017/06/07/performance-improvements-in-net-core/.Net Core is also Open Source so it cannot be shutdown because a company decided to do so.
The only downside is that existing project will need to update the Framework version they target so they can use the latest SFML.Net. Also not every .Net Framework API have been ported, but i would say that only very obscur/not used a lot/windows related APIs are missing (the only one that was really missing for the SFML.Net conversion was introduced in .Net Standard 2.0 and it's a very specific API to manage memory ressources if i remember correctly)
Another "downside" is that OpenTK is not on .Net Standard yet (they are planning to in the future) it's not really a direct downside because it's not something we maintain and we still expose everything so it can work, but it will prevent interaction with OpenGL until then.
Also Microsoft is really showing that .Net Core is the new Standard for .Net, so we kind of need, in my opinion, to offer a .Net Standard library if we want to stay relevant in the future.
Finally a good place to get more info is this forum post where we discuss if we wait for .Net Standard 2.0 or not
https://en.sfml-dev.org/forums/index.php?topic=22150.0And the actual pull request
https://github.com/SFML/SFML.Net/pull/143