The GraphQL spec supports introspection of args

 type __Field {
-   args: [__InputValue!]!
+  args(includeDeprecated: Boolean = false): [__InputValue!]!
 }
 
  type __InputValue {
+   isDeprecated: Boolean!
 }

This allows clients to continue to see deprecated input args so that things are deprecated but not broken.
Additionally, this can be used by clients to ratchet out usages of the deprecated fields - ensuring no new usages of them

Context:

• Allow deprecation of input values (field args, directive args, input fields) graphql/graphql-spec#805