If you are running over the source key limit, you can consider deleting deprecated keys. However, be sure that deleted keys are no longer necessary for your project, as you cannot restore them if needed, and all the associated data is also deleted.
It's not only about the translations you can easily store and restore if needed when using deprecated keys instead of deleting them, but also about context information, comments, user interactions, etc. Once the key is deleted, it's no longer counted against the key limit, and it's impossible to restore it.