This study supports the argument that  WHOIS might not be an essential tool for fighting spam and the world did not see a massive spam apocalypse as it was said it will (it is "a" tool but not essential and most probably replaceable even). Yes it is a short time frame and might not be complete but it is at least based on some data. If there are other studies that provide data that can support the argument that WHOIS is an essential tool for fighting spam (other than all the statements  and individual cases) and as a result of compliance with GDPR and redacted personal data of domain name registrants there is an increase in spam, then we can look into them. 

I would also like to note that domain name hijacking has decreased according to some registrars as a result of redacted personal info in WHOIS (as was said during our call a couple of weeks ago). Domain name hijacking is an issue domain name registrants especially in developing countries had to deal with for long with almost no recourse nor remedy. They did not have the resources and the knowledge of using UDRP or lodge transfer complaint to get their domain names back.

Privacy in WHOIS might even be good for cybersecurity and protecting trademarks. Why don't we look at it this way too. Not implying that we should not address all other issues that can be raised because of redacted personal information from WHOIS, just saying WHOIS is not that dark because of protection of privacy of domain name registrants.