By the time the MCU ended its third phase, it had managed to release 23 films to form a cohesive story comprised of nearly all the elements that came before Avengers: Infinity War and Avengers: Endgame. Those movies worked off the ones that came before, but that doesn't mean there wasn't a ton of foreshadowing building up to the inevitable conclusion, pitting the Avengers, Guardians of the Galaxy, Wakandans, and more against the combined might of Thanos.
Some eagle-eyed observers noticed these bits of foreshadowing as well as those foretelling events in their own movies as the films were being released. Over time, more than a few folks had a good idea of what was coming with each successive MCU release. Fortunately, those viewers have shared their observances via the MovieDetails subreddit, making it possible to look back and have an "oh yeah" moment from any of these interesting examples.