It's not few. It's a very common mistake. Say I have 20 10G ports on line card. Mid size ISP has from hundreds to thousands routes. Thus we will end up with tens of hundreds ACLs per line card. That affects performance, dramatically. Say on older yet supported Cisco 7600 you can apply about 10k entries per port, ~ 100k totaling per card. That's limit, after that performance degrades. Another question what to do on peer-peer links, like say NTT<->Centrylink? Both on them have interconnects across the globe ad sending each other hundreds of thousands prefixes on 100G ports. That's the challenge as well. Hardware is not there, it's coming slowly but not yet there. Most of the big players DO one or another form of ingress filtering on some ports. Valid point is that smaller players are arrogant ob the issue and dump all all the garbage to the upstream.

Incase of huge DoS attacks they do something similar. But with DDoS attacks the spoofed packets are coming from so many different locations that it is hard to identify them, and often it is just stuff like compromised toasters.

I am not this kind of network engineer, HOWEVER. Both IPv4 and IPv6 are versioned by the first 4 bits of the packet. Depending on that value the address size and location are fixed.

I would not consider the comparison of the source address of packets crossing an ingress link to be 'deep'. I consider that check to be very shallow. It needn't even be every packet from a set, merely picking a random (actually random) packet and testing for conformity is a good quality control measure that SHOULD be taken.

What would the comparison be against? Routers are supposed to know which links are on the other side of all down-stream connections so that they can effectively route.