The mode of communication depicted by the Duffer brothers shouldn't be able to be as accurate as it was in the first season of the show. Since the lights and the letters would not have been visible for Will to see in The Upside Down, and even though he can validly communicate through lights, it would have been impossible for Will to see which letters Joyce had painted on the wall corresponding to a particular light that he also couldn' The flashlight can be tapped by the unseen to blink, off, or even dim in response to questions.
The discovery of lights, dimming and turning off was one of the earliest forms of communication to The Upside Down.
The lights are ok in this example because they seem to be triggered when a human is near the lights and the corresponding real world location. A lot of what the team uses to communicate does in fact have legitimate scientific roots in the paranormal community as actual methods of research, and are therefore valid methods of communication that one might be able to use from one perspective to another. The Upside Down is stuck in the past by four years, but the lights are a valid form of communication. The method of using lights has been a mainstay of communication and general location throughout the show since Joyce noticed the lights acting oddly. The lights and the letters would not be painted on the wall in the present time. Joyce paints letters in the present, corresponding to a light on the wall, after figuring out that Will can communicate using lights.
Before that occurs, though, audiences were left with new information regarding The Upside Down and some of its inhabitants, as the origins of both unfolded a little more throughout a terrifying season 4. This has not been the case for the question of how Will Byers communicated with The Upside Down in the first season of Stranger Things.
The stage is set for the final season of the hit show, as the last two episodes have come and gone. Trigger items can be used to welcome a spirit to communicate using the 'yes-no' method. The location of season 4's big bad, Vecna, was determined by the members of the Hawkins gang using lights. Joyce tried to answer the question of 'Why' the entire season after her magnets fell off her fridge. The Duffer brothers appeared in an interview with Variety to say that there would be an answer to the issue in Season 4. This also has scientific accuracy, as paranormal investigators often useEMF to find and locate spikes of electromagnetic influxes to determine a spot where a spirit may manifest.
The method that Will uses in season 1 that could potentially create a plot hole is lettering. The kids' compasses are not pointing true north, and that's when datememe datememe datememe came into the picture. The world will be watching the Battle for Hawkins. Will Byers used this method to communicate with his mother in the first season of the show.
The fact that communication from the other side is possible is still an important plot point. It would not have been accurate to say that WIll was spelling out small words and sentences. A spirit can use a triggering item, such as a flashlight, to acknowledge they are present.