There’s a bug in the iOS app:
Settings > Push Notifications > option Comments on Others’ Activities > disable
This doesn’t turn off notifications for comments on other people’s activities - I keep this disabled but once I comment on someone else’s activity, I am still being notified about new comments.
This makes me think twice before e.g. congratulating someone on a great achievement as I don’t want to be hit by a surge of notifications that I’m not interested in. I’d be happy to comment more and encourage fellow athletes, but this bug is stopping me.