Enumeration of the entire DNS space is not available in general, but it does appear that some TLDs offer complete zone files for legitimate research purposes, see for example https://czds.icann.org/help#zone-files

> apparently, "asking DNS servers for all the domains they know about" is not something you can really do anymore for security reasons. Guess that idea won't fly lol

There are actually a few nameservers that will just give all their domains to anyone who asks [0], but they are very much in the minority.

[0]: https://github.com/acidvegas/mdaxfr#tlds-that-allow-axfr

You could probably extract a lot from https://commoncrawl.org/