�And Now She Does Porn
So here's what I think happened:
To combat slipping ratings, the producers of Family Matters decided that there would be a horrific boiler accident in which little sister Judy was monstrously disfigured, receiving burns and shrapnel wounds to over 98% of her body. She would die days later during Sweeps. The event would be so tragic that the Winslow family would decide to never speak of Judy again and to act as though she never existed.
The network censors would not let the episode air because it was too disturbing, but the producers decided that the show's change in format would stay intact anyway. The absence of this episode made it seem as though Judy simply went upstairs and never returned, but, in reality, her disappearance was entirely thought out and not at all half-assed.
And that's why, some time in 1993, Judy Winslow ceased to exist.