Halo Infinite has shaved off another feature that Bungie-era games shipped with, but to be fair, the game doesn't necessarily need it.
343 Industries and Microsoft have confirmed that Halo Infinite will not have online co-op matchmaking. Yes, the game will still have online co-operative play allowing you to connect with up to 3 other friends and play the campaign. But it will not feature the matchmaking system originally introduced in Halo Reach.
To be fair, Halo Infinite doesn't need this feature. Halo Reach's online matchmaking was introduced in an era where players didn't have things like Discord, which recently came to Xbox. Bungie also introduced rotating campaign challenges in Halo Reach allowing you to earn credits to unlock armors and cosmetics. Campaign matchmaking was (and still is) a nice bonus feature, but 343i has a lot more dire issues to worry about in Infinite right now (like the data bug, the massive desync issues, or even monetization).
"Online matchmaking will not be available with final co-op," an Xbox representative told GamesRadar.
"We encourage you to continue to use the Halo LFG and the new Discord voice call feature on Xbox to find players to party up with as you continue playing the beta."
Also let's not forget that Xbox has a built-in Looking-For-Group (LFG) feature that easily lets players team up with other players. PC has Discord which is also useful, and the official Halo Discord will likely have a section devoted to campaign co-op LFG players.
So while it would be nice to have online co-op matchmaking in Halo Infinite, it's hardly an important or necessary feature like it was back in the 2010 Halo Reach days.