They're working on it as of 2 months ago. It seems like currently they are tuning a system to more accurately detect afk and troll activity before they start to implement more severe punishments. It's a fine line to walk though. If someone has a baby that wakes up in the other room and they need to leave the game, should they be banned for the rest of the day? If it happens again the next day, should they be banned for a whole week? It's a mobile game, and sometimes life happens.
Conversely, I was playing Yuumi yesterday and my ADC scored an ace. Because I didn't need to actively poke or switch to other players, the most effective thing to do was stick to my ADC and heal for the attack speed bonus. 30 seconds later I got a warning about being afk, so I started spamming my q while the ADC shot at turrets. 60 seconds later I was informed that I would receive punishment for being afk. If the punishment was more severe than a 20 minute ban, I would have been pretty upset.