We discussed this at IPC earlier this year in October, and Derick asked to me open a ticket; here it is:

XDebug currently can report which lines of code are executed (used for code coverage). I often face the problem of having to find unused code in large codebases. This currently can only be done by exporting lots of data from Xdebug and then "diffing" them against "all lines of code in the codebase".

If xdebug could optionally export a list of code lines that have not been executed, finding unused code would become much more feasible (and have less runtime impact).