ZFS has a 'utf8only' property which, in principle, lets you constrain all filenames on a dataset to be utf-8... only. It does not otherwise change the API, but

ZFS has a 'utf8only' property which, in principle, lets you constrain all filenames on a dataset to be utf-8... only. It does not otherwise change the API, but it should make the creation of a non-utf8 file into an I/O error. This defaults to off, so by default you're right. Apparently the GP found a bug in this code. I'd be interested in seeing the github issue.