In 1998 there were fewer than 5000 different networks (Autonomous systems) on the Internet. Today that number is over 80K. Network operators used to personally know each other and their clients.

So the automatic filtering is better today than it was in 1998, but it can still be easily bypassed.

I know for a fact that even as a tiny network, all you need to do to get the some of the biggest networks in the world to accept your (real or fake) BGP announcement is to email their support center.