Because Homebrew was originally a package manager that built everything from source and even today has to fall back to building from source every once in a while, our packages normally install everything that is needed to develop software with a given package. Thus *-dev packages like in Debian are not needed. In your particular example, you'll find the headers in $(brew --prefix)/opt/unixodbc/include or /usr/local/opt/unixodbc/include (if you have installed Homebrew in the default location).

Thank you, this is both excellent and very helpful. I wonder if there is some way to explain this in the running software or the documentation. It might already be there, though I looked around for quite a while before entering the issue. Thanks again for your answer.

I wonder if there is some way to explain this in the running software or the documentation.

I'm not really sure what “this” is referring to, so feel free to provide a bit more context. But generally you can have a look at brew info <formula> and it will list all installed versions of that formula, including the paths they are installed to. Everything belonging to a certain version of a given formula is located in its prefix, that you can learn by issuing brew --prefix <formula>. The Formula Cookbook primarily addresses formula authors, but also starts off with some terminology and some overview of relevant formula paths in Homebrew. (Maybe that's the documentation you were looking for?)

By "this", I mean that I searched everything I could find about homebrew quite extensively, looking for information about headers or "dev" packages. I could not find anything. Perhaps I overlooked it, but my suggestion was that the information you gave above, which is that brew already automatically provides those files and therefore does not need any extra packages, is fantastic information that I wish had been easy to find and obvious when I was looking for it.