It’s difficult to pinpoint when exactly the decline started. But one key event before the Settings app was the Catalyst apps that were straight out and dismal ports from their iOS versions. Till date, none of those work well and cannot be navigated properly using the keyboard. Reminders, Messages, Notes and more.
Craig Federighi seems to be increasingly taking on so much authority without having a trusted set of people under him and his leadership (or lack of it) has resulted in neglecting software across device platforms. Some of the Apple apps on tvOS with paid subscriptions are worse, because the bugs in them don’t get any attention at all.