> Enforce scope (namespace) requirement, and require external verification (reverse DNS for example).
Who the heck says everyone who publishes a library has a domain? That seems absurd.
You can get subdomains for free from a number of places, some of which are more reliable than others.
This exists because domains (historically) used to be expensive by western standards. .com used to be $75/year back in the day.
Why don't you? It costs around $20 per year. Every serious computer nerd should have one, and a web server with at least a basic homepage.
Sonatype allows "io.github.<username>" as a valid groupId and has a process to verify ownership. I am sure other providers like GitLab can work on this.