Even now fillet brazing has it's place. As has been mentioned a large number of custom builders still braze and it is also useful for repairing frames that have cracked in the heat affected zone of their tig welds!
Modern tubesets have been developed that react well to the high heat of tig welding because tig welding is cheaper. Not because tig welding is better. I am not claiming a degree in metalurgy, but as someone who brazes I can guarantee that a properly brazed joint has plenty enough strength for a bicycle frame.
As for production frames that are brazed, modern Pace frames such as the RC 104 are brazed and as mentioned Saracen did a brazed Kili for 1989.
And not made out of shabby tubing either.