It isn't a server API or anything... It's just some code in chrome, and it has to remain because it's still allowed to be used in Manifest v3 but for a narrower scope of usecases.

(Note - I'm specifically talking about the controversial WebRequestBlocking part of the changes - the rest of the manifest v2 to v3 migration is largely just like a python 2>3 migration - there is equivalent functionality for everything).