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**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](https://writing.codidact.com/questions/tagged/1/terminology), 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?".