How do I flag a method as deprecated in Objective-C 2.0?

IMHO, it's easier to write __deprecated:

- (void)myDeprecatedMethod __deprecated;
- (int)methodNameDeprecated:(int)param __deprecated;

Works too on classes

__deprecated
@interface MyDeprecatedClass

  // ... some properties and methods ...

@end

If you want to give additional message with the deprecation flag, you can use following flags.

@property (strong, nonatomic) NSString *catName
                    __deprecated_msg("use name instead.");

//  -- Or -- 
@property (strong, nonatomic) NSString *catName
                    DEPRECATED_MSG_ATTRIBUTE("use name instead.");

//  -- Or -- 
@property (strong, nonatomic) NSString *catName
                    __attribute__((deprecated("use name instead.")));

Using above mentioned flags, you can tell why you are deprecating or what is the method developer should use in future.

To mark a method as deprecated, use __attribute__((deprecated("Your message goes here")))

A practical example, from Mantle

@interface NSValueTransformer (UnavailableMTLPredefinedTransformerAdditions)

+ (NSValueTransformer *)mtl_externalRepresentationTransformerWithModelClass:(Class)modelClass __attribute__((deprecated("Replaced by +mtl_JSONDictionaryTransformerWithModelClass:")));
+ (NSValueTransformer *)mtl_externalRepresentationArrayTransformerWithModelClass:(Class)modelClass __attribute__((deprecated("Replaced by +mtl_JSONArrayTransformerWithModelClass:")));

@end

Deprecation Syntax

Syntax is provided to mark methods as deprecated:

@interface SomeClass
-method __attribute__((deprecated));
@end

or:

#include <AvailabilityMacros.h>
@interface SomeClass
-method DEPRECATED_ATTRIBUTE;  // or some other deployment-target-specific macro
@end