The reason why it was originally introduced:
arduino/Arduino@a6909bd

Why we are removing it now?
* Windows does preserve the state of the RTS/DTR bits on successive
  opening of the serial port.
* The serial library used in the Arduino IDE 1.8.x has a bug when trying
  to set DTR=false, on successive opening of the port the DTR line is
  set back high by the USB serial driver. This works differently from
  the serial library we use in the Arduino CLI, that sets DTR=false for
  good and this change is preserved on the successive opening of the
  port.
* Having the serial port left in a state with DTR=false may cause
  problems to tools uploading later.

It may probably completely removed, but for now, to reduce the testing
surface, it will be disabled only for Windows.

Co-authored-by: Cristian Maglie <[email protected]>