i think this is unnecessary (not just saying it because i'm a stone rank) just like @zRinne said here
As long as you're not responding to in game interaction (and standing still at that) then it's most likely because the player is AFK, there isn't a need for it but if it does get added, they should add they auto-reply message when someone privates chat you only. No need for public messages because not everyone would need/want to know you're away.