Anything less than being able to track memory lifetime at compilation is GC.

Some folks will jump up and down and say "variable sized allocation is sloppy!"

GC is a spectrum on which reference counting lives. Is there an arch that supports bidirectional references?

Maybe linear types actually, affine is not enough?

I suspect affine is fine, but I'm not an authority. The distinction (as I understand it) is only that linear types require that you consume all instances, whereas affine allow you to leak if you want. Leaking doesn't seem to lead to dangling pointers, but I've been wrong before.