I have a 2018 Focus ST Line X which I bought earlier this year. It never seems to be away from the dealership. It’s been in for a new bonnet release (bonnet opened when driving but not fully - caught on the first catch ), the keys weren’t recognised the other week and it’s in again today with what I thought was an issue with the rear seatbelts and a fault with the start/stop.  
I was driving with three in the back all belted up. Dropped off one and the warning light on the dash illuminated with the ‘beeping’ as I drove off. Had to plug the belt back in to stop the warning and the beeping. This also happened when I dropped off the other two.  Had to drive home with all belts plugged in.   Went out the next morning and drove with none plugged in and no warnings.   The dealership have just called to say it’s not a fault but apparently a safety feature.  They’ve said it’s not something they’ve been aware off (said they had to phone round other dealers and Ford HQ for an answer) but it’s to warn the driver that one of the belts have been undone especially with kids in the back and the only way to reset it is to switch off and restart.  
Am I being fobbed off or is this a genuine safety feature?

Assuming you have a MK4 they may be correct. I don't know if the MK3.5 is different.

As far as I know the rear seats do not have occupant load sensors so rely on the driver hitting the ok button after start up to confirm the number of seat belts used. If one of the rear belts is taken off and the car has no way of knowing that a rear seat has been vacated then there will be a warning.

If that is how it works, and I don't doubt you, then it is Quite a poor system if it does not allow the driver to override things by ok'ing the number of seats occupied again without restarting the engine.