Along with Joseph Quinn, Eduardo Franco was missing from the Stranger Things season 5 cast photo Netflix shared. Franco's exclusion is much more notable than Quinn's, though. At the end of season 4, Argyle made the cross country roadtrip with Eleven, Will, Mike, and Jonathan. He even drove them there in the Surfer Boy Pizza van!
In a video from the Steve Varley Show, Franco is asked if he's in season 5, and he mentions that he didn't get invited to be a part of the final season. Here's what Franco said:
"It’s nice to hear that there’s some sort of concern or something, you know what I mean, but I never got a phone call, so yeah, I think that's it. "
There's still a lot of time to bring back Franco and Argyle if the Duffer Brothers want. It sounds like Franco is open to returning, but it's all about how the character fits into the story. Maybe, with as many characters as there are in season 5, it's just not doable. We don't know.
If Franco has not been contacted yet, it is a bit concerning given the fact that the scripts are likely done. The Duffer Brothers usually write and rewrite as they go throughout production, but they've had a lot of time to get these scripts to where they want them, much more time than they had in the past between seasons, so we have to assume the scripts are done and Argyle will not be in season 5 at this point.