Why? I have a hard time imagine a use case where restoring the OS itself would be appropriate.

I can imagine restoring data, obviously, and services running with their personalization ... but the OS is something generic that should be discarded at whim IMHO. You probably chance few basic configuration of some services and most likely that's stored in /etc but even then 99% is default.

You can identify what you modified via shell history, e.g. history | grep /etc and potentially save them or you can also use find /etc -type f -newerXY with a date later than the OS installation and you should find what you modified. That's probably just a few files.

If you do back up anything beyond /home (which should be on another partition or even disk than the OS anyway) you'll most likely save garbage like /dev that might actually hinder your ability to restore.

So... sure, image the OS if you actually have a good reason for it but unless you work on archiving and restoring legacy hardware for a museum then I doubt you do need that.