$> git annex --mumba 2>&1 | grep -e url 
  addurl          URL ...                   add urls to annex
  rmurl           FILE URL ...              record file is not available at url
  registerurl     KEY URL                   registers an url for a key

$> git annex version | head -n 1
git-annex version: 8.20210223-1~ndall+1

So ATM there is addurl/rmurl when operating on a FILE, and there is registerurl when operating on a KEY, but there seems to be no way to unregisterurl as to rmurl from a KEY. I have not found how could it be possibly done ATM without direct editing of .web files in the git-annex branch.

edit 1: well, instead of adding unregisterurl could be done by adding --key flag to rmurl I think.

unregisterurl done --Joey