I made a pull request for updating the z3 opam package here https://github.com/ocaml/opam-repository/pull/15087 and was told that the latest version of z3 could be installed via the flag --dev.
z3 has recently switched a dependency over to zarith from num. It was suggested that an opam file should be in the z3 repo
I don’t think it makes sense to put an opam file at the root of the project. Can I add the opam file for a package not at the root of the project?
Well, you are allowed to put your package definition files in a opam/ subdirectory as well 
(quoting the manual: « When pinning a package, the source is searched for metadata in an opam or <pkgname>.opam file, either at the root of the source tree or in an opam directory. »)
Out of pure curiosity, what makes you think it “doesn’t make sense”?
Thanks for the replies. They are very useful.
I don’t know what the maintainer’s opinions is, I still have to ask, but on subjective aesthetic grounds, I wouldn’t want an opam file in the root directory of a project whose main focus is really C, C++, and maybe python. It’s kind of messy from that perspective. Maybe I’m totally off base.