Can I use a restore image based on DietPi 8.25 to provision a new Pi running DietPi 9.3 or do I need to maintain an up to date restore image?
If you have an (backup) image of a former installation you can flash it over every version you want.
The automated restore on first boot is for backups made via dietpi-backup. And if you are connected to the internet it will update to the latest DietPi version on first boot.
Thanks for the clarification!