I think this is a very good idea. It can be very confusing to have a tag that appears valid; yet when used, your question is down-voted and/or closed.
We do need to be able to inform users when a tag is not to be used for questions. The kind of message @yoda suggests is informative and to the point. There should also be some way to keep new career-dev careerdev tags from being created by a user with > 1500 rep. I would also assume that any synonyms of career-development would auto-magically throw the same message.
My point for supporting this feature is that there are many tags that will eventually become off-topic and/or outdated. No one is willing to provide a mandate because that may trigger adverse deletion, removal, or migration of questions. While the career-development tag exists without any form of notification that the tag is now off-topic, it appears to be a legitimate, on-topic subject. As it stands now, off-topic and out-dated looks the same as on-topic and out-dated - e.g. .net-1.0.net-1.1.net-2.0.
The mechanism that the OP suggests prevents:
- additional questions added to otherwise bookmarker tags
- off-topic questions posed to incorrect site
- needless confusion when said off-topic question is down-voted and/or closed