So you have received a combined total of 2 Activity Challenge Notifications (if not in US then ONLY 1) in over 600 days of use that you are not "fine ignoring" and so far have made 2 posts about it typing even more words than were in the original 2 Notifacations. How on earth do you deal with multiple daily email spam Notifications (several 1000 since the

Watch launched)?
Also there is NO way to opt out of the 2 Notifications. However if there was a feature it would require you to open the Watch app, click on Notifications, Click on Apple Activity Challenges (or whatever it would be called) and then toggle off the switch. Seems rather excessive when all you had to do was simply ignore the 2 Notifications sent. Hopefully for your sake Apple will introduce an opted out feature before the next one drops which could happen as soon as July 4, 2017 (US). But if not then mentally prepare yourself because you WILL have to 'deal with it' by ignoring it.
If 'dealing with' and ignoring something as
completely benign and infrequent as the 2 Activity Notifications is that problematic then you SHOULD consider becoming more active to help lower the stress Apple is causing you.
Becoming more active directly benefits your quality of life (especially latter in life), your family and indirectly benefits society as a whole.