Forge mode will also be missing at launch.
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343 Industries that they are delaying some fairly major parts of Halo Infinite for some unknown time after launch. The developers say that they want to focus on getting the single-player campaign finished along with the competitive multiplayer. Keep in mind here that Halo Infinite was originally supposed to be released during the 2020 holiday period but was delayed for about a year to a still vague holiday 2021 release window.

Head of creative Joseph Staten made this announcement today in the August video update on the official Halo YouTube channel. The reveal begins around the 16:40 mark in the video below.

343 says that they are "100% committed" to getting Halo Infinite out this holiday season.

Forge is a sandbox style game mode where players can create custom game modes, custom maps, custom rulesets, and more. Some of the modes created in Forge have even gone on to become official online modes for Halo games. I believe the popular Grifball mode began life as a Forge creation from Rooster Teeth back in the day.

Campaign co-op has been a very popular mode in the Halo franchise. Co-op in the Halo franchise began with two players and was later expanded to supporting four players all working through the campaigns together. Its omission from Halo Infinite's launch is a pretty significant loss.

Staten says that they will take a seasonal release schedule for adding in campaign co-op and Forge to Infinite. the game will launch with Season 1 for the game's multiplayer. Staten says that the team at 343 will try to release campaign co-op in Season 2 and Forge in Season 3. As a new season is expected to be released every three months, it can be assumed that Halo Infinite will not get campaign co-op until early 2022 at best. This is assuming Halo Infinite comes out around November 2021.