Marathon has been delayed to an unknown date

This was to be expected, and Bungie decided to make it official this Tuesday. Marathon isn’t coming out on September 23 for PS5, Xbox Series and PC.
With a lengthy statement, the American company explained the reasons behind this delay.
"The Alpha test created an opportunity for us to calibrate and focus the game on what will make it uniquely compelling—survival under pressure, mystery and lore around every corner, raid-like endgame challenges, and Bungie’s genre-defining FPS combat."
"We’re using this time to empower the team to create the intense, high-stakes experience that a title like Marathon is built around. This means deepening the relationship between the developers and the game’s most important voices: our players."
"Over the next few months, we’ll continue closed testing (including participants from the Alpha) to deploy gameplay updates and test new features as they come online."
Bungie intends to focus on three main areas of development: the survival aspect of the game, deepening the Marathon universe, and creating more social experiences.
At the moment, Marathon does not have a new release date. The studio plans to communicate about it as early as next fall. It is quite possible that the title could slip to 2026.