In its most basic and simplest form, filtering access to content on the web can be achieved by rather blunt instruments such as DNS black-holes. And, in the early 2000s, this was more than enough.
Over time, though, the web became more popular and useful, both for users and attackers, and web filtering providers needed to up their game again: simple URL filtering was used to allow access to example.com/news and block access to example.com/badstuff. Again, for a time, that was enough.
