Knob and Tube wiring - is it a dealbreaker?

Saw a few houses Oakwood-Vaughan area that have ~25-50% knob and tube wiring on inspection reports. I've been told by my realtor that this is a huge issue for insurance, and that most major insurance companies won't touch it without it being replaced.

My question is...if the majority of older houses in the area are knob and tube, how did they get insurance? Was it just grandfathered in?