• %n is not supported in bionic. [1]
  
• Microsoft disables the %n format specifier by default. [2]
• dietlibc does not support %n. [3]

Discussion

Arguments in favor of the proposal:

• Instead of using %n we can always use the return value of one of the printf-style functions.
• Despite being standardized %n is not provided by several C-libraries, thus cannot be used in portable programs anyway.
  
• If the format argument of a printf-style-function originates from an unverified source an attacker may be able to write arbitrary values to the stack.
• %n is used rarely, with one major exception. It is often found in code fragments demonstrating and/or performing stack attacks.
• Deprecating %n gives a grace-period to those programs still using %n.
  

Argument against the proposal:

• If a program uses untrusted strings as format specifier for a printf-style- function, it is unclear if there is really a security gain by not supporting %n. Using an unverified format specifier from an untrusted source is not only a local error but also a strong indication of a sloppy programming style ignoring security entirely. In addition it might well be possible to form an attack with other format specifiers e. g. by using %s.