“Measurements indicate that the DNS Cookies do not provide sufficient coverage, even though around 40% of nameservers by IP support DNS Cookies, these account for less than 10% of overall query volume.”
The fix Google just turned on by default comes from a March 2008 draft. Google has run 0x20 case randomization since 2009, but only against a small set of nameservers. It now covers over 90% of its UDP traffic to nameservers. The biggest public resolver on Earth needed 16 years to ship a trick that costs nothing but flipping letter case.