There is little support for the term "item" applying to intelligent creatures
Player's Handbook
In the PHB, the term "item" appears 85 times, and nearly always refers to equipment, or is used in the context of magic items, which are objects (they can be sentient in some cases). There are very few exceptions:
- Mounts and other animals: a table on page 157 lists creatures such as Camel, Pony, or Warhorse under the header "item". It however does not include creatures with higher than animal intelligence. (This fits the definition as an item being anything on a list).
There are only a few uses in spells where there is some ambiguity. For example:
Telekinesis allows you to manipulate creatures or objects, and allows you to retrieve "an item from an open container". I think most DMs would rule this would allow you to retrieve a creature from a container, not just an object.
Mage hand allows you to manipulate an object or retrieve an item from an open container. It is not clear if you could use it to lift a tiny creature, but the majority consensus on this site is that you can. If so, the situation is as with Telekinesis.
Dungeon Master's Guide
In the DMG, the term appears 784 times (mostly because of long lists of magic items, and wondrous items), and practically always refers to some kind of object. There are only very few magic items that use the term in their description (other than referring to themselves or to magic items in general); for example:
Bag of holding states that retrieving an item requires an action.
Heward's handy haversack likewise states that retrieving an item from the haversack requires you to use an action.
Quiver of Ehlonna refers to items, but it is pretty clear these are meant to be ammunition only.
The Haversack and the Bag are exactly what spawned this question, because living creatures such as player characters can be put inside: both of them talk about how long a creature can survive within before suffocating, which would not make sense if you cannot put creatures in. But if
- you can only remove an item from them, and
- player characters cannot be considered items
How do you then get the PC out again? Can they somehow climb out by themselves, even though the items do not say this is possible? Will they just suffocate in there eventually unless they can plane shift or banishment themselves back out?
Conclusion
There appears next to no support for "item" referring to a intelligent, living creature in the rules. So a player character would typically not be considered an "item". In a very few cases, spells and magic items will work in unexpected ways if the term is not allowed to be used with an extended meaning of "object or creature"; it will be up to the DM to decide how they adjudicate such cases.