For example, let's say we implement support for a web standard or vendor prefix. We can mark it as deprecated. But how do we know the code path is no longer i

For example, let's say we implement support for a web standard or vendor prefix. We can mark it as deprecated. But how do we know the code path is no longer in use? Are people still using this CSS property (e.g., a vendor prefix)? Are people still using gopher or this one configuration variable? The more configuration options you have, the more combinations you need to test and maintain.