I was actually writing a completely unrelated feature request and while writing it I noticed it's already possible, however I noticed that SO doesn't actually do anything at all if one of my tags doesn't have any questions in it anymore. Here's what I'd propose as possible solutions for cases such as these:
Provide UI feedback when tag is deprecated
In this context deprecated means depleted out of questions. In this particular case, the tag
postgresql used to have a bunch of questions but apparently they had been all retagged under
postgresql obsolete/deprecated. It would've been nice for the UI to pop up a notice about this so that I could've replaced the tag as soon as such action became relevant instead of now after for how long this has really been the case.
Add a way for users with high enough karma to redirect tags
By allowing users to create automatic replacements to deprecated tags SO could support automatic changing of the deprecated tag to whatever it should really point to. This way only one person or a relatively small group of persons would really need to do the change and it would propagate itself to the entire user base.
Allow for metaranking/-linking of tags.
There may be completely valid reasons to keep some of the tags but as far as I'm concerned, I wanted to notice the
postgres questions even though I had selected
postgresql. If we are allowed to metalink tags (once again, by user with respectably high karma), the system could automatically help me manage my tags with the Google-esque "Did you mean someothertag?" type of feedback message or some other non-intrusive method.
UPDATE @ 16 Nov 2009
Well, now someone has changed them the other way around again! Or maybe some time ago, can't really tell, this is extremely annoying!