It appears a new Halo game is in development — separate from Halo Infinite — according to a new job listing on 343 Industries' website. The developer is seeking a Producer who will “help develop a new project in the Halo universe.”
As GamesRadar aptly points out, it's highly unlikely this “new project” is referring to Halo Infinite, as 343 is too far along in the development cycle to be seeking a Producer.
The Halo series is no stranger to spinoffs and side games, so there is precedent for multiple projects to be in the works simultaneously. Examples include Halo Wars, Halo: Reach, Halo Spartan Assault, and Halo Spartan Strike — all of which are separate from the main numbered entries in the series.
The job listing doesn't specify what kind of game this new unannounced project is, or really anything else that would hint towards what it could be. Since 343 is seeking a Producer, the project is likely very early in development, so we shouldn't expect to learn more about it for a while.
Over the past weekend, 343 Industries stated that there would be “a new place and way to play” Halo: The Master Chief Collection, though whatever this is referring to will roll out later this month (or early next month). Because of that, the Producer role is likely not tied to the new Master Chief Collection update. The community has speculated that the collection will come to Epic Games Store or possibly Nintendo Switch, but nothing of the sort has been confirmed yet.
Halo Infinite is the next mainline entry in the series and it's set to launch sometime in the fall of 2021 after being delayed multiple times. It was initially supposed to launch alongside the Xbox Series X|S in November 2020, but 343 opted to push it back a year to ensure it meets fans' expectations.