I help often customers understanding technical debt, and code quality. Most people focus of code metrics, but they tend to forget organizational metrics, and how that impact the software. There are several studies on this. I particularly like one from Microsoft [1].

One of the metrics that I use address your point:

Number of Ex-Engineers (NOEE): This is the total number of unique engineers who have touched a code and have left the company as of the release date of the software system

Implications: This measure deals with knowledge transfer. If the employee(s) who worked on a piece of code leaves the company then there is a likelihood that the new person taking over might not be familiar with the design rationale, the reasoning behind certain bug fixes, and information about other stake holders in the code.

A large loss of team members affects the knowledge retention and thus quality.

[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...