So Johnson died at the end of Halo 3 after Guilty Spark lasered him to death. But with his return in Halo Wars 2, does that mean that he is still alive in the Halo universe? Or did 343 just add him onto the game for fan service?
According to Halo Waypoint:
Even with the small handicap of being dead, Johnson isn’t one to disobey a direct order from his beloved Corps and he's sprung back to "life" ready to kick all butts that need kicking.
Reading this, it just sounds like 343 added him into this game. Although many may be happy that he is "back" in this game, it is entirely unlikely that he will make a return in any of the main series of games since it would also likely upset any fans of the franchise to just see a main character return out of the blue (but that's just an assumption).
Furthermore, Sergeant Forge died during Halo Wars (the first one), but is also in Halo Wars 2 "magically."
Johnson does not make any appearance in the story based campaign for Halo Wars 2 and is only present in the distinctly non-canon Blitz and Skirmish modes as a possible leader along with other characters that are very much dead in canon by the time the campaign takes place e.g. Sgt Forge and the Arbiter that appeared in the first Halo Wars who both died on the shield installation shortly before it's destruction.
Halo Wars 2 Takes place during/after the events of Halo 5. In lore, Sergeant Johnson is dead, and that is undeniable. 343 just added him as fan service, in a DLC, however, if you are fond of the idea that Johnson could still be alive, here is a suggestion:
Remember when Johnson was brought back from the Dead in Halo 2, since in Halo 1, it was presumed that he died in Installation 0