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
Answers get outdated, that's for sure. That's why we ask that answers stand on their own, without needing to link to other site.
That particular answer, however, should be flagged as "link-only" answer. It is a perfect example where an answer become quickly invalidated when the link expires. If you write a solution to a question in the form of an answer, you need to provide enough information to make sure that other people can replicate your solution without needing to visit your link.
tl;dr Generally, just leave a comment. In this particular case, flag that answer as "link-only" answer.