Similary in answer for this question … if You don’t want to mess with any kind of 1×1 transparent image, this work’s too:
[[UITabBar appearance] setBackgroundImage:[[UIImage alloc] init]];
[[UITabBar appearance] setShadowImage:[[UIImage alloc] init]];
In swift:
UITabBar.appearance().shadowImage = UIImage()
UITabBar.appearance().backgroundImage = UIImage()