For example, Notes app is not listed on the Apple Product Feedback page. And I want to suggest adding a Horizontal Line, like HTML
<hr/> to Notes app.
After suggestion from @user3439894, I had tweeted Apple Support and here is their answer:
We love feedback. If you don’t see the Apple app you wish to leave feedback for, you can leave feedback by choosing the device you are using here: http://apple.co/Feedback
So, following that, I had posted the feedback using one of my devices as a channel, explaining in notes why am I using the device channel, and also listing all the OSs that I mean this feature request for.