Given that it's very hard to find anything about dependency version ranges in the official documentation (the best I could come up with is http://docs.codehaus.org/display/MAVEN/Dependency+Mediation+and+Conflict+Resolution), I wonder if they're still considered a 1st class citizen of Maven POMs.
I think most people would agree that they're a bad practice anyway, but I wonder why it's so hard to find anything official about it.
They are not deprecated in the formal sense that they will be removed in a future version. However, their limitations (and the subsequent lack of wide adoption), mean that they are not as useful as originally intended, and also that they are unlikely to get improvements without a significant re-think.
This is why the documentation is only in the form of the design doc - they exist, but important use cases were never finished to the point where I'd recommend generally using them.
If you have a use case that works currently, and can accommodate the limitations, you can expect them to continue to work for the forseeable future, but there is little beyond that in the works.