Has been moved to diagnostics repo. The specs have been moved to documentation/symbols and the code to src/Microsoft.SymbolStore and src/Microsoft.FileFormats
The latest builds are still available on the dotnet-tools feed:
https://dev.azure.com/dnceng/public/_artifacts/feed/dotnet-tools/NuGet/Microsoft.SymbolStore https://dev.azure.com/dnceng/public/_artifacts/feed/dotnet-tools/NuGet/Microsoft.FileFormats https://dev.azure.com/dnceng/public/_artifacts/feed/dotnet-tools/NuGet/dotnet-symbol