I'm not sure this, as suggested, would be a good idea.
I fully understand what you're getting at, but I think it would have undesirable, and also likely unintended, consequences.
This is a slightly contrived example, but I hope it'll illustrate my point. Writing Codidact has a
terminology tag (here, in case you're curious). That's a tag that could probably apply on a number of sites covering a variety of subjects.
Now suppose someone wants to post a question that would appropriately be tagged
term (which doesn't happen to exist on Writing specifically, but also isn't completely unreasonable as a tag).
If we implement what you're proposing, the person posting that question won't be able to tag it
term simply because a tag named
terminology already exists, even though
terminology might not apply at all.
There are probably better examples, but my point is: disallowing creation of substring-match named tags can prevent perfectly reasonable tagging.
If anything, it'd probably be better to prompt the user before the question is posted along the lines of "you are about to create a brand new tag
a to categorize this question; do you want to do that?".