NO, THEY DID NOT FORGET ABOUT WILL'S BIRTHDAY....
as we know, Mike arrived at California on March 22, 1986. As it was mentioned in season 2, this date is supposed to be Will's birthday.
People pointed this out, saying that Will was mad at the Rink O' Mania because everyone forgot about his birthday. But the Duffers denied it and said it was a production mistake and that it would be fixed. (They forgot, but not the characters)
but some days after that, via twitter, the stranger writers account said that no scenes have been and would ever be edited on the show.
Plus, they have already made some references to forgotten birthdays before...
like, the duffers were insane for this one
-> early birthday reference robin & vickie
so, a very popular theory (which I agree with) says that, Vecna took away this memory from Will (his birthday date, which Joyce has mentioned), and all the other memories that were mentioned to him during his posession (rainbow ship, mike talking about the first day they met, etc).
It would make so much sense knowing that Will is the key of everything this next season. He's the main target now. Vecna wants him.
Check -> will byers has powers theory
Even if that's not the case, there's something we know for sure. Will's birthday day is coming back in season 5!
As we know, there's a great possibility that Will is going to relive the Rink O'Mania events in the first episode of season 5.
He may probably be in a trance or a vision where he is trapped again in a bad memory. In this case, the Rink O' Mania one. We know this because Noah recorded some scenes there using his finale outfit.
the first one is his finale outfit and the second one is his March 22nd's one. We haven't seen him at the Roller Rink with any other outfit than his blue shirt.
But he was there with his final outfit!
"but actors were still spending time in other sets even if they were not in those scenes". Yeah, that's a good point, but not this time. The Roller Rink has been rented for over a month for this.
You would not rent the place so much time just to film 10 minutes of the show there.
We also had this spoiler! (noah's typical behavior)
He was wearing a harness under his finale outfit, but we haven't seen any scene that requires one (yet).
Still hesitating about Will having a vision? Then what was Lonnie's actor doing in season 4's production BTS? (i'm afraid this bit*h is also part of Will's bad memories :c)
Finally, the final scene script also revealed that s5 continues right where season 4 has been left. Something will interrupt the scene and I may have and idea of what is...
-> another theory related to this, is Mike's unsent letter about Will's birthday?