The buildopts.arches field was added to the modulemd spec:
https://github.com/fedora-modularity/libmodulemd/blob/cc59dea337d585c14b67568263b31e4939de9586/spec.v2.yaml#L249

This needs to be supported in MBS. This should be accomplished by setting the arches on the Koji build tag MBS creates for the module build. The arches should only be a subset of the arches that the Platform/privileged module sets. If they are not, we need to either raise an exception or record a warning that would be shown in the /module-build-service/1/module-builds/<id>/messages API endpoint.

See this issue for more context https://github.com/fedora-modularity/libmodulemd/issues/399.