There is no doubt that the climactic battle in Avengers: Endgame is one of the most iconic moments in the MCU as we saw all of our favorite heroes in the MCU up to the point in an epic battle against Thanos and his army. Since then, the event has been referenced in several Phase Four films and TV shows including Ms. Marvel in its first episode.
According to Marvel.com, during the AvengerCon sequence in the first episode of Ms. Marvel, you can spot in the background a sign for the book I Was There... written by a SHIELD agent who claims to have witnessed the climactic battle in Avengers: Endgame.
The book details "the clash between the heroes of earth and the Alien scourge threatening our fragile planet." It also includes "a conversation with Hawkeye, and a first-hand account of Tony Stark using the Infinity Gauntlet (even though the author was 6,000 feet away)."
You can see the sign on the left side of the screenshot below:
This is a confirmation that there was a SHIELD agent who was somehow present during the battle and was able to keep the details of what happened during the event. However, it is a bit jarring to hear this since it's hard to imagine that an agent was present in that moment since the entire area was a battlezone and we only saw our favorite heroes (who mostly have powers) during the sequence.
Perhaps it's all just a fun easter egg that Marvel added in the scene and maybe the book is not exactly as accurate as it claims to be. While we as fans and viewers know what actually happened in the battle, the people in that universe don't exactly know what happened except for the surviving heroes who were present and participated in the battle.
New episodes of Ms. Marvel premiere every Wednesday on Disney+.