TL;DR We’ve moved the F# GitHub repository from microsoft/visualfsharp to dotnet/fsharp, as specified in the corresponding RFC. F# has a somewhat strange history in its name and brand. If we roll back the clocks to the year 2015, F# sort of had two identities.

情報源: The F# development home on GitHub is now dotnet/fsharp | .NET Blog

微妙な二系統の開発は止め、.NET Coreベースのオープンソースプロジェクトへの集約という事らしいです。