>From a conversation in Libera's #debian-python channel today, it sounds like making pybuild's pyproject plugin backwards compatible for writing data_files to the root filesystem is unlikely to happen. I've filed a bug[1] against the upstream cloud-init project to make the required upstream changes to support PEP517 on debian-based distros. More context can be found there.
[1] https://github.com/canonical/cloud-init/issues/5027
