Do you need a codex to play Warhammer 40k?

You do not actually need the Codex: SM for Blood Angels. They will eventually get their own codex, but in the meantime you could either use the Index or the Codex until the chapter specific book is released.

Do you need a codex for kill team?

Will one need Codices for kill team? No need for codices. Kill Team does not use the 8th edition rules. It has its own rulebook, which includes faction rules.

What is a Warhammer 40k Codex?

A codex (often pluralised as codexes by Games Workshop, though the grammatically correct pluralisation is codices), in the Warhammer 40,000 tabletop wargame, is a rules supplement containing information concerning a particular army, environment, or worldwide campaign. 'Codex' is now a term solely used for army books.

Are normal space marines being phased out?

No they aren't. Not yet anyway. All standard marine kits are still on sale, none have been replaced by Primaris kits. Also, in the background, normal marines are still being produced alongside Primaris marines, up to the current point in the Dark Imperium timeline.