Watching “Loki” on Disney+, it provides a content warning: “Some flashing lights sequences may affect photosensitive viewers.” I remember when Incredibles 2 was released in theaters, it caused someone to suffer an epileptic seizure, and there were soon warnings all over the theater about it.
But how do they determine that people with photosensitive epilepsy might have an adverse reaction to the flashing lights sequences, before the content is even released? Surely they don’t have someone in the room with epilepsy the whole time, and check a mark if he suffers a seizure!
Flashing lights are known to trigger seizures in some people with epilepsy. So if you’re going to show a movie that features scenes with flashing lights, it’s a courtesy to give a heads-up to epileptics that they might not want to watch it. It’s not necessary to wait until a viewer actually has a seizure before putting that warning up there.
As Machine_Elf stated, flashing lights can trigger a seizure in one who is susceptible to having them. There are other triggers, including auditory ones, that can have a similar effect. In fact, strobes are sometimes used clinically in order to induce a seizure. Playing video games is another method of purposely causing a seizure.
It can be weird. There was a famous case years ago where someone had a seizure upon hearing a specific television personality’s voice (Mary Hart, one-time host of Entertainment Tonight).
I personally knew of someone whose seizure trigger was hearing Phil Collins’ singing voice.
If all this is true, why didn’t they have warnings for Incredibles 2 until someone actually experienced multiple seizures while watching the film? I’m not epileptic and even I could tell that that particular sequence could probably be hard to tolerate.
Because there’s no law that says any movie or TV show has to have these warnings. It’s become increasingly common, and I’m sure at this point for some studios it’s Standard Operating Procedure, but not every single production is going to have someone attached to whom it occurs to include this sort of warning.
Many disability advocates, including the Epilepsy Foundation, have raised concerns that movie scenes with flashing lights, including that in Incredibles 2 of Elastigirl’s fight with the Screenslaver, can trigger seizures in viewers affected by photosensitive epilepsy.[89][90][91] As a result, several theaters posted warnings for audiences.[92] Disney told USA Today that it appreciated those efforts, and then, in a memo, asked all theaters exhibiting the movie to warn audiences: " Incredibles 2 contains a sequence of flashing lights, which may affect customers who are susceptible to photosensitive epilepsy or other photosensitivities."[93]
In response to this, a re-edited version was released in the United Kingdom with all affected sequences altered so that any flashing lights and strobe effects now pass the Harding test.[94][95]
In other words, there was a change of policy between 2018 and now, which is why “Loki” includes the warning you described.
That web page includes a link to the Harding test, which apparently is useful in determining whether a given video clip has the potential to induce epileptic seizures:
I think that provides a pretty definitive answer to your OP.