Tag Tracking has been in the manual since at least 1.0, the experimental bits are the StrictEffects, 'effectsOf' and 'forbids' stuff which build on top of machinery that has been a part of Nim since the beginning.
So, when used as I've done in my toy example, it's not experimental. In particular I'm not relying on strictEffects or forbids to prevent compilation, I'm only relying on effects bookkeeping machinery.
