Leave a comment under the answer
That would be enough to inform future reader who is interested in the tool that it is no longer available.
That would be enough to inform the community to downvote the answer as in current state the answer is wrong
That would be enough to inform the answerer that the link gone broken, and whether they want to fix it or not
Should a post like this be flagged for moderator intervention?
No, it shouldn't be flagged just because the link is broken, moderators are already overloaded.
What else you can / should do:
flag it for other reason if you find a valid one - as pointed out already in the previous answer
down-vote - sure, sooner or later there will be enough to move it ...