hey eXpl0it3r
I've been using SFML.NET to develop a game for the last few months and will be continuing through to late next year so despite being a little late to this discussion i'll provide my two cents.
As someone who is still using .NET Framework I will confirm SFML should be moved to .NET Standard as this assures its compatibility with the various CLR platforms (.NET Core and .NET Framework). For as long as SFML.NET only relies on whats available under .NET Standard there shouldn't be any compatibility concerns and means you don't necessarily need to keep a .NET Framework branch for legacy reasons.
A note to why i'm still using .NET Framework (other than its what SFML currently provides) as it may clear up some concern for this migration.
.NET Core the main CLR platform at discussion here was initially made for console application development in the cloud which really doesn't fit with where SFML gets used. As a result certain namespaces for UI and basic graphics/imaging in .NET Framework aren't available in .NET Core without 3rd party support.
I so happen to be using these namespaces (e.g WinForms and Drawing namespaces) and cross platform isn't a big priority for me at this time.
I want to point out the upcoming .NET Core 3 looks to potentially add what i'm using anyway to help those such as myself move from .NET Framework to .NET Core as that is the preferred platform choice for the future.
https://blogs.msdn.microsoft.com/dotnet/2018/05/07/net-core-3-and-support-for-windows-desktop-applications/I can't think of any other real reason at this current time why you might stick with .NET Framework and so its reasonable to see SFML.NET move forward as a .NET Standard library.