Unused code paths were once used or it was thought that they would be used. If you don't know if something is used, it's safest to assume it is being used or is there for some reason (Chesterton's Fence). If it's being used, you need to maintain it lest there be a regression.

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.