Schemas grow and change over time. GraphQL values backwards compatibility quite highly, so changes to a schema are typically additive: introducing novel fields, types, and enum values.
However, when the implementation of a field is just wrong, it can be kept for compatibility, but deprecated.
The value for the key can either be
true, or a string description of the reason the field is deprecated.
Typically, the description indicates an alternative field to use instead.
Deprecation does not affect execution of the field in any way; the deprecation flag and reason simply shows up in introspection.