Cannot use "air" in /replaceitem

Verification builds: Release 891562
Summary: air is not allowed in the item argument of /replaceitem

Impact: It is literally impossible to remove an item by slot name from a block or entity, such as removing the weapon from a monster.

Repro Steps:
1. Hold an item in your hand.
2. Run /replaceitem entity @s slot.weapon.mainhand 0 air

Observed Results: "Syntax error: Unexected 'air' at 'ainhand 0 »air«"

Expected Results: The item is removed.

Screenshots/Videos attached: No.

Note: This is not a new issue, or blocking live content.