It’s always possible that they really did just forget Will’s birthday, human error happens all the time, but I will say that emphasizing Will’s birthday in a previous season -> writing the birthday out on the screen -> throwing in a few other lines about birthdays and forgetting them in the same season -> building an entire storyline in the same season around forgotten memories -> throwing in a scene about Will not remembering something from childhood very well -> Will’s birthday being forgotten becoming big on social media and the Duffers playing into it -> claiming that they were going to edit their show to change his birthday -> later stating on twitter that they will never edit their show -> continuing to bring it up + making a birthday post for Will on his birthday is a very very funny turn of events if the writers really did honestly just forget