You should probably not completely nuke this work from history and it should be clear for anyone else what this prototype is about and what to do about it. So, you have to keep and document it.
For example, if you have a developer guide, a bug tracker, a todo list, ... add an entry for that branch to keep a reference about the feature. Later, someone can check through the unmerged features and decide again if they are worth integrating. Alternatively, or additionally, use...
It can make sense to prefer having branches only for actively developed work and use tags for trees that are not going to change anytime soon. So if you replace your branch by an annotated tag, you (1) remove a branch and (2) have an opportunity to document it:
git checkout branch
git tag -a unmerged/featureA
# Write in editor
git checkout master
git branch -D branch