Didn’t have time to explore this just yet, but I think another way to collect info about executed / not executed code could be running a production release with coverage information enabled 
As claimed in OTP 27 release notes, :cover module that offers this functionality just got shipped with noticeable speed improvements
Could be risky / expensive to run this in production though, so must tested first
I have code running every now and then. It would require to collect info for months before removing anything 
It doesn’t look like your mix unused project you referred to here was ever posted to this thread. Here’s a link to the project:
Strictly speaking, QA team are employed to ensure all production functionally is valid on every release, so if they are doing their job the :cover approach should be valid based upon QA test cycles. This could be an interesting way to validate that said team are doing their job.