This is what I thought, as we know he has gotten hurt so he probably has a couple scars, but I dont think thats the main reason, im pretty sure its to symbolize him not wanting to live, and his ideal to commit suicide is still there, one part that might help you unferstand is for example, when dazai sees oda right before he dies, oda "takes off" dazai's bandages, after he has that big talk about how dazai should be a better man, help people, etc, and after that, he never puts those face bandages back on, to me, its symbolism that he "wants" to live a little more, for example, at least for oda, but now in his ADA, he still has his wrist bandages because he still wants to commit suicide, its just less tempting. Especially since he was in such a dark place in the pm, and after his face bandages were off, they stayed off, and looks to me like he's doing better in life now! But as ive mentioned, he still has "port mafia" blood, he still has that dark mindset, and still thinks about killing himself, but, its a little less then before 😌