In the workspace, there are planning and budget constraints. For example, much of Java's Date api is deprecated (because it was horrible), but that's something that's hard to replace gradually. And I can't just replace it in the whole massive codebase without getting budget approval, so it doesn't get fixed for years.
In my hobby projects, I tend to fix all warnings including deprecations almost immediately. But few of those projects ever get finished. Perfect is the enemy of done...