Hi Peter,
I've found some relevant documentation for this conversation and wanted to share it in case anyone else had this question.
"Start time, end time, and ad schedules will be unavailable for the Asset-based Structured Snippet extension, and will not be automatically migrated."
In the same paragraph, it is mentioned that the remaining Advanced Option for device preference ("Mobile") will be honored, but not be available in the future.
I think if it will be honored in the short term, it makes sense to keep the field in for a while.
Thanks,
Kevin