Not a HUGE deal, it just means you can't leave your PC for too long. You can still AFK train and go watch TV and go to your PC during commercials and just change tabs really quickly.
The fact is Jagex purposely implemented this update and kept it hidden. There was really no reason why they needed to stop AFK for various reason.
1. AFKing results in no money gained and less experienced earned per hour
2. AFK is dangerous and risky depending on location and player stats
3. Certain monsters remain aggressive from a certain periods of time
4. Legit players can easily killsteal AFKers
5. No harm done to the game