It depends, just same as normal JS packages.
Generally it should be peerDeps if you publish a library, otherwise put into dependency if you build app.
More specifically, peerDeps describes requirements for dependencies to be hosted on end-user site. It’s mainly needed for toolchains such as plugin systems, compilers, and meta-frameworks.
It is also the ReScript module system expects, because even if there are multiple dependency versions, your package manager may chooses one main path, and ReScript internally rely on that assumption.