Don't recall ever hearing it got mothballed during development - though I do believe they've bent over and are going to support csproj files with .NET Core in the future as well.

Here's the relevant page on project.json: https://docs.microsoft.com/en-us/dotnet/articles/core/tools/...

Edit: I need to be better about deep reading through links. https://blogs.msdn.microsoft.com/dotnet/2016/05/23/changes-t... looks like project.json will be mothballed. Kind of.