Find unused translations

Using a module/script
You can use the script https://github.com/locize/locize-lastused​
This module/script will update the last used timestamps on reference keys (so only on your reference/source language).
Use this only during development or in staging. It is not recommended for production usage because of the additional workload and http requests done.
Check out the going to production section.​
Using the API
Like the script you can also use the API.

Both fill the last used date which can be filtered in the UI:
Depending on your needs it should be safe to remove keys older than a few months.
Last modified 10mo ago