By the definition of that class, anything that is not an object with injected dependencies is legacy code. That's a rather bold statement ;)

In hooks, it is usually fine to use static methods on the Drupal class. Making that a service like suggested there is only useful if it's non-trivial code that someone might want to implement differently or/and you want to write unit tests for it.

If you're writing a service, plugins, form classes or controller classes, you should inject the dependencies.

So it's not really a question about which methods you should be using or not, it depends on where you would call it.

By the definition of that class, anything that is not an object with injected dependencies is legacy code. That's a rather bold statement. ;)

In hooks, it is usually fine to use static methods on the Drupal class. Making that a service like suggested there is only useful if it's non-trivial code that someone might want to implement differently or/and you want to write unit tests for it.

If you're writing a service, plugins, form classes or controller classes, you should inject the dependencies.

So it's not really a question about which methods you should be using or not; it depends on where you would call it.