Specifically this section.

Not that this guide is a black-and-white list of rules, but pretty much everything in it was discussed before it went up, so that was the general opinion at the time... er, a year ago? Whenever I actually wrote the thing?

Anyway, there is absolutely no reason I know of that you can't spork a fic that's been discontinued for other reasons. Most fics aren't finished. Go right ahead :)

Wasn't it more about the author attempting to remove the fic than discontinuing it? In most cases even after an author tries to remove a fic it's still archived somewhere, and I think it was considered kind of mean to spork a fic that was at least attempted to be taken down.

If it's clear the author wised up in the middle of a fic, it sort of ruins the climate for sporking it. "THIS MARY SUE IS SO BAD... oh wait. It just self-terminated."

A fic I sporked got discontinued because the author returned the game it was based off of and didn't want to finish it, or even finish the game she was playing. If that doesn't betray how little care the story got, then I don't know what does.

A lot of Suefics are never finished anyway: merely forgotten and never updated.