Files that were not modified in the last 6 months. This can occur in two cases:
1) Developer who was working on this file has left the team;
2) Developer who was working on this file has stopped contributing and moved on to carry out other work;
Why is it dangerous?
Developer who haven’t touched the file in such a long period no longer remembers how it works. It will take time recover this knowledge. More time should be allocated in case changes need to be made in such files.
While this is a natural lifespan cycle of the product, having a large portion of codebase in lost knowledge can point to an inadequate team size for the project.