Although s390x is a supported architecture (along with x86_64, ppc64le and aarch64), kata-containers not available for that platform.

In brew, the latest RPM is indeed built for all platforms described above:
https://brewweb.engineering.redhat.com/brew/buildinfo?buildID=2417305

But, looking at the RHCOS releases page, taking the latest image build for instance:

• x86_64: https://releases-rhcos-art.apps.ocp-virt.prod.psi.redhat.com/contents.html?stream=prod/streams/4.13-9.2&release=413.92.202303161330-0&arch=x86_64
• s390x: https://releases-rhcos-art.apps.ocp-virt.prod.psi.redhat.com/contents.html?stream=prod/streams/4.13-9.2&release=413.92.202303161330-0&arch=s390x
• aarch64: https://releases-rhcos-art.apps.ocp-virt.prod.psi.redhat.com/contents.html?stream=prod/streams/4.13-9.2&release=413.92.202303161330-0&arch=aarch64
• ppc64le: https://releases-rhcos-art.apps.ocp-virt.prod.psi.redhat.com/contents.html?stream=prod/streams/4.13-9.2&release=413.92.202303161330-0&arch=ppc64le

Is it some configuration that explicitly says the sandboxed-containers extension is only available for x86_64?

We are particularly interested in having the extension available for s390x, it is a requirement coming from IBM. ddepaula@redhat.com and jfreiman can provide more details about this request.