Develop a tag schema that can be applied to the classes and methods and scripts in the various codebases and leaves evidence in artifacts such as logs from production.
Then analyze that data to know what's called when and how often.
You could do this for a subset of traffic and/or a subset of time to get a good sample.
Not a complete answer to '...
Mutation testing (or mutation analysis or program mutation) is used to
design new software tests and evaluate the quality of existing
software tests. Mutation testing involves modifying a program in small
https://pitest.org/ for java
https://pypi.org/project/mutmut/ for python