We have a couple of challenges today. Hobbyists often go over 20MHz, because they put WiFi, BT or USB on their boards, giving EMC issues. Also, the speed of the modern ICs tend to be very high. If you have a 9600 Hz UART signal, that is not a 9600 Hz signal if it's a square wave with a modern IC with very short rise time on the pins. So a good old, slow serial line can with modern MCU emit noise up in the hundreds of MHz range.
So your PCB layout tips are important, even on slow circuits these days.
Depends on your definition of beginner. It's trivial to put a nrf52 module (like [1]) in a PCB design and wire a USB socket to it; just make sure to route the data lines as a differential pair in kicad (add protection diodes if feeling fancy). And speak a little prayer that it actually works as intended. No need to understand what any of that means.
Of course the notion of using such a module might be a step up from beginner for you, but IMHO it's more about the understanding. But I agree that there is no definite definition.
So your PCB layout tips are important, even on slow circuits these days.