A question was asked that requested a a certain style of answer which I attempted because it exposed the difficulties in a way that interested me. Subsequently the question was edited and the constraint removed. My effort was wasted and the question became bog standard. Is there, or should there be an audit trail for changes to answers?
You can see a full edit history of a question and of its answers. Doing anything more would needlessly increase the complexity of the site
Generally Speaking, We prefer that question askers not fundamentially change their questions in a way that can invalidate existing answers.
On the flip side, If someone asks an awkward question, it's usually best to get clarifications in the comments first than to post an answer that you're unsure about.