All those involved with the Pokemon anime have made sure to stay far, faaaaaaaaaaaaar away from showcasing Porygon after the infamous seizure incident from years ago. For those who don't know, the 38th episode of the Pokemon anime, entitled "Cyber Soldier Porygon," featured a series of strobing lights that lead to 680 children in Japan having seizures, or seizure-like symptoms. The episode was obviously taken off air immediately, and unfortunately for Porygon, the character disappeared from the series as well.
Those involved with the Pokemon anime didn't want to feature Porygon, as it would bring up too many bad memories for the team and viewers alike. That said, it has been over 20 years since that episode aired. I guess that's enough time to move on, and start to make some sneaky references to the Pokemon. As you can see in the tweet above, there was a direct nod to Porygon, even if the Pokemon itself didn't make an appearance.
I say Porygon has suffered long enough. The only other appearance of Porygon after the incident came from a quick cameo in the Pokemon 4Ever movie intro, and that was it. It's time to let Porygon be a part of the fun again!