An ancillary feature that doesn’t affect core gameplay: A want.
An issue that, while vexing, is not missing.
A game mode that is somewhat absent (I say this because Slayer is still present in Bot Bootcamp), and is ultimately a want (Yes, it’s been in all previous games but is not necessary to core gameplay; for further clarity this is not an argument that it should stay out, only that it is a want)
Same as above
Core features would be the entirety of the Campaign (yes, co-op is a want as well) or Multiplayer as a whole. I would even consider the Multiplayer to be toeing a line, but that’s just me. The presence of vehicular combat, a variety of guns with distinctive roles, and a variety of alien enemies. These are Core Features, and details further extend into wants.
In the context of what I said, I fundamentally disagree. Core features in the context of Halo is the baseline that all games since 3 have had at launch (except halo 5 which launched in a similar state).
That means actually functional custom games, forge, campaign coop, and multiplayer playlists & gamemodes. The absence of these do 100% prevent gameplay because the gameplay is different in coop, in customs, in forge where you can spawn vehicles & weapons at will. The game is missing vast amounts of its baseline content to be added at a later date. That’s not GaaS - that’s early access.