I don't think there's anything wrong with stashes, but there's another interesting trick that you might want to try for this purpose: commit the dead-end, then

I don't think there's anything wrong with stashes, but there's another interesting trick that you might want to try for this purpose: commit the dead-end, then immediately make a revert commit. You can even add a rationale in the revert commit message. The technique is not applicable all the time, but for those instances when you really think it's valuable to preserve the knowledge that was gained in trying and abandoning a particular approach it's great.