Originally, Avengers: Infinity War did include Adam Warlock, at a much earlier stage, but he was cut.
Phase 4 of the MCU may very well bring us Adam Warlock, but just don’t expect to see him in Avengers 4.
Even though the character didn’t appear in Avengers: Infinity War, many thought he may show up in Avengers 4 next year.
Speaking to the possibility of Adam Warlock showing up in Avengers 4, Joe Russo made it crystal clear that he’s not going to show up in the movie and explained their reasoning for excluding him from the story.