after sae asks yourself to turn yourself in to testify against shido, akechi shows up to turn himself in instead, which is where the new month comes in since you dont go to jail, everything after that is hard to summerize without explaining a bunch of shit but basically akechi surviving in the first place was due to the new antagonists power and he helps you stop him knowing well that he would vanish akechi doesnt try to make it look like he is a matyr either, but the other characters are a little sympathetic (depending on how high your link with akechi was I think) Its kinda hard to describe but the vibe I got definitely wasnt that he was "redeemed" more just that he was fully accepting of his punishment
Okay, that seems better, I figured they would just ignore/erase his actions.