I was not aware that he has retired from the ODI format, he will now focus on league cricket. Since T20 is a short format retired players can play without any issue. MSD is still playing league cricket after he retired from all formats of cricket. Yes, he is undoubtedly one of the best opening batters that Australia has produced the good thing was that he was part of the World Cup winning team. He might be replaced by Marcus Harris that is what is being said by the Australian media.
David Warner is 37 years old and it's best time to leave test when you are at peak of your career. There are lots of T20 leagues going on around the world, I guess he will go for IPL only as other leagues are not paying much. Marcus Harris doesn't have an impressing test career, in his 26 test innings he wasn't able to score any century so far.