Floating traces on MCU dev boards bad for noise?
It’s okay to leave I/O pins unconnected if you ensure that they’re programmed to have a pull-up or pull-down, or are programmed as outputs.
What you don’t want is a floating input. That can lead to unstable operation and excess power draw.
That said, if proximity to a noise source is a problem, you can bypass the (pull-up/down) pins with small capacitors.