Denied
It's unimportant who reported you. You aren't allowed to go afk. You should tell all the players why you have to go and then save the replay, but even that won't ensure you get unbanned. It is your responsibility to make sure you are able to finish a game before you join it.
20 days is a rather short penalty for being booted with negative score in a 5v5 game. I won't reduce it further.