Can someone explain why it wouldn't work to have porn companies use only certain domains, and filter based on those domains, if people are so intent on blocking it?

The smart version of this is that adult content providers send a header or something with every response that contains NSFW, and then you use parental controls on client devices to not show those responses. This even works for sites that show a mix of kid-friendly and adult content cough reddit cough. It offers fewer opportunities for general purpose censorship, though, so there's less interest.

The US Congress evaluated [1] the legality of forcing adult websites onto certain TLDs in the 2000s and it was determined to be legally ambiguous and would almost certainly face substantial 1st amendment challenges. So they didn't pursue it.

https://www.everycrsreport.com/files/20080714_RL33224_1e6b93...

Because you shouldn't get behind a thing that shouldn't exists. Technical solutions to societal problems are and will always be naught and fraught with unnecessary jump roping. In this case, the content itself isn't dangerous, what is dangerous is consuming it without the correct context, ie. education.

What's the enforcement mechanism if some porn company in Elbonia "forgets" and uses hothothot.el, instead of hothothot.porn?