After taking Armour Thyroid for 6 months at 120mg/day for borderline subclinical hypothyroidism, would it likely be safe to stop cold turkey?

Check with md. It should not be a problem, but at that dose i would go a bit slow instead of stopping altogether at once to make sure your own thyroid "wakes up" and starts to produce thyroid hormone. If you had hypothyroidism (not subclinical) i would be cautious about this as you can become ill stopping thyroid hormone that your body needs (called myxedema).
Check with your MD. Starting and stopping thyroid medications should be under the direction of your doctor. Check with them to see if stopping medications is an option for you.