From an online article:

When [Rod] Serling started the opening credit narration in 1959, he initially said that there were six dimensions, but to avoid backlash, they changed it to five. “There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and timeless as infinity. It is the middle ground between light and shadow, science and superstition, and it lies between the pit of man’s fears and the summit of his knowledge. This is the dimension of imagination. It is an area we call the Twilight Zone.”

Backlash over “six dimensions”?