Just because Ronin may have been active up until a year ago; he may have been very good at keeping his identity a secret and not made many public appearances. Remember it was James Rhodes who was able to track him down so that Natasha could confront him. Just because we the audience see him, doesn't mean the public necessarily knows about his activities in every single instance.The show says that Ronin has not been seen in years, but it is now supposed to be only a year after Endgame. When was Clint recruited in Endgame?