I respectfully disagree with you. There are in fact crawlers whose job is to continuously crawl domains, resolve the IP and log the DNS info. In fact, it’s a huge market.
If you use Cloudflare properly, no one should ever know your servers true IP. Hence, it’s security through obscurity. I could give you a domain which has properly used Cloudflare and even the class B subnet the server is on and I doubt you will be able to correlate the two, unless of course, you slip and create an A record which has the servers true IP in it, which will get logged very quickly.
Use discourse itself as an example, how do you think this passive DNS is available?
https://viewdns.info/iphistory/?domain=discourse.org
I digress, the intent of this topic is derailing. If you would like to discuss passive DNS, crawlers and opsec. I will be happy to do so in another category/thread.