I do have a little in-app banner at the top of the logbook screen for messaging like prompting to create an account, or upselling premium, or other things. It has grey background, but its buttons are blue and it has a red dot on it so it doesn't look to clone the appearance of a system notification perfectly or anything (it isn't even pinned to the top of the screen or anything, where notifications come from). Maybe you thought it was a notification? But it can only occur in-app, on one screen, and is in no way a violation of Apple's ToS.
Edit: Screenshot of the in-app banner to aid discussion: https://www.dropbox.com/s/i3ib5pbfg5eid51/Screen%20Shot%2020...
Edit 2: The banner used to be all blue, which looked even less like a system notification, but I was worried it was too distracting from users' content so this season I opted for a grey background to help it blend in more.
Sorry for the kerfuffle, I mean no harm to you, your reputation & your app. I wish I had screenshotted that. I remember almost reaching out to you on Twitter about it, but thought "hey it's the first one I'll let it slide". I vaguely remember it about the premium subscription, which I found odd. I was not pissed, just surprised. Was it push or local notification? I can't say if I can accurately tell them apart, in my opinion a notification is a notification. It was definitely not in-app as unfortunately I won't get to ride this season so I haven't opened the app in a year :( It was around Christmas or New Year IIRC
Also, I used Slopes as an example because it's the most recent one but frankly it's far from the egregious stuff others pull, especially Apple and the few other developers mentioned in this thread.
I don't use local push notifications for advertising, either. I use them for A) location-based recording reminders created by users and B) notifications when a recording from the Watch is synced to your phone and ready to view, since that can take a minute or so if you're off wifi.
I remember one person reaching out shortly after the iOS 13 launch that I trigged the "Slopes has been using your location in the background" notification when I wasn't recording, which was an error on my side and I fixed that right away.
I did have an email or two go out around Christmas for a giveaway, so I certainly had some email marketing in play then. Maybe Slopes appeared in a notification from your mail client? But Slopes itself did not trigger any notification, push or local, for that campaign.