The Pokémon franchise has been huge from the get-go, but one early anime episode nearly ruined the series' momentum right off the bat, and the end result was Porygon getting banned from appearing ever again.
One common criticism of anime, particularly back in the '90s, was that it was often low frame rate, since each frame had to be drawn by hand--an expensive and tedious process. This sometimes resulted in choppy animation, which is where a lot of stereotypes about anime in Western culture come from, but there's a bigger problem that came about as a result: the capacity to trigger seizures in people with photosensitive epilepsy. While it wasn't the first anime to have this issue, the Pokémon anime became the source of one of the biggest incidents of epileptic seizures of all time, as a result of one now-banned episode.
On December 16, 1997, the episode "Electric Soldier Porygon," the 38th episode of the anime, was aired on Japanese television. In the episode, Porygon takes Ash and co. into cyberspace, where they find that Team Rocket have done the same with a different Porygon, and used their presence to install a virus that could intercept Poké Balls in transit before they can reach their destinations. As is typically the case, Ash and Pikachu fight Team Rocket, and at one point, Pikachu launches a thunder shock to protect their Porygon from being mistaken for a virus itself. In destroying the antivirus missiles aimed at Porygon, a series of frames flashing between red and blue light effects is shown, lasting about 4 seconds. While the frame rate may be low from a film perspective, it was at just the right speed to cause seizures, and sent nearly 700 Japanese children to the hospital. The anime went off the air for months as a result, and every previous and upcoming episode was edited to reduce any strobing effect that might occur. "Electric Soldier Porygon," however, was never shown again.
Porygon Did Nothing Wrong
The episode's airing and subsequent aftermath was understandably a major incident, and sent Nintendo's stock tumbling afterward from the bad PR. Despite the fact that Pikachu's attack was to blame for the strobing effect that actually caused the seizures, Porygon became the scapegoat, never to appear in another anime episode (outside of brief, occasional cameos). Even when Porygon received new evolutions, those Pokémon were banned from the anime as well, meaning Porygon2 and Porygon-Z have never appeared at all. The original episode was also never released dubbed in any other language as a result. In modern episodes, flashing effects like Pikachu's electric attacks are significantly slowed down and dimmed, so much so that it's easily noticeable for anyone aware of the series' history.
Porygon's unique computer-related abilities give it a plethora of opportunities for plots that wouldn't be possible with any other, and Porygon itself is a fairly popular Pokémon whose presence has been missed. With the incident nearly 25 years in the past, perhaps the time has come for the Pokémon anime to make up for lost time and feature Porygon again.