--gc:arc/--mm:arc has been around for a couple years (as per that blog post link) and fairly stable for >1 year. It is slated to become the default automatic memory management strategy in Nim-2.0. The hope is to release 2.0 this year.

You can always make it your own personal default with older versions (e.g. nim-1.6), by editing your $HOME/.config/nim/nim.cfg to say so or doing similar on per project/file basis.