The reason for signal not going back to LOW
The pin is not configured as OUTPUT.

By default, all of the GPIO pins on pretty much any microcontroller are configured as inputs following reset. This prevents the chip from "fighting" an external device that might be driving any of those pins before the firmware has had a chance to configure the GPIO correctly.

On the MCU used on the Arduino, the output latch controls the state of an internal pullup resistor when the pin is configured as an input. Your code was simply enabling and disabling this resistor. When you set the pin low, it makes the pin open-circuit, and you see a slow decay of voltage through your oscilloscope probe.