We had to make do with 75 baud - but we were fortunate compared to those who had even lower bit rates. It was easy to jam the Teletype keys by typing too fast - even with two fingers.
IIRC the lowest I heard about was 1 baud. That was a low power radio rescue channel transmitter. The receiver had a very, very narrow filter. To keep the transmitter frequency stable it was designed so the downed pilot's armpit acted as a crystal oven.