It's funny, I think we all seem to take it for granted that every single Marvel character that has appeared in the MCU—and is still alive—will show up in 2018's Avengers: Infinity War, but that may not be the case. The Spider-Man issue is a dicey one because Marvel continually has to go back and work out deals with Sony any time they want the character to show up in one of their movies, which makes it so that even the actor playing Spidey, Tom Holland, has no clue whether or not he's going to show up in a film.
While speaking with Variety recently, Holland let us know that there are near constant negotiations going on between Sony and Marvel, and he just sort of hangs out until they tell him where to show up. When asked if he would be appearing in Infinity War, Holland responded, “It’s all up in the air. I believe some sort of deal is in the mix, but I’m unclear as to what that deal is.” Honestly though, could you imagine Spider-Man not showing up in Infinity War? It seems like an impossibility, but this is a business first and foremost and no one's appearing in anything until the suits determine how much money they're going to spend and/or receive in order for this to happen.