Halo Studios has explained what Halo Campaign Evolved players will need to play online.

They clearly stated that everyone will need to have a Microsoft account, even if they’re playing on PS5. This is the latest Xbox game to have such requirements on Sony’s platform, following Grounded and Sea of Thieves.
If you want to play online Halo Campaign Evolved, you will also need to have an Xbox Gamertag, even on PS5.
To clear up any confusion, there is splitscreen local co-op on Xbox and PlayStation, but each player needs to have a Microsoft account, and for PS5 players, a PSN account linked to their Microsoft account. Online on Xbox Series X|S and PlayStation 5 requires subscriptions to Game Pass or PlayStation Plus respectively.
These arrangements are necessary for Xbox to enable cross platform and cross play for their game across PS5 and other platforms. While PS5 gamers are happy that they will still get Halo Campaign Evolved in their platform, they should have already known that they would have to deal with this too.
