(edit: Regarding Ctrl-Alt-Del) That might depend on whether you have "Save on Embark" enabled, or seasonal/yearly saves if it actually takes that long to decide you want to start over.
I often make a copy of worldgen dir (prior to any playing at all) as "regionN_raw", just in case I want to have another go at the same site but shuffled over by a tile or so, or adventure/fortress in a parallel setup without my original fortressing/adventuring having affected things. I don't often use it, but I know it's there if I want it.
(You should be able to recreate all significant terrain features by taking the various random seeds of your original worldgen that can be found in one place or another and explicitly using them in a new worldgen to get the same map and significantly similar (if not identical) in every other aspect of history, I think. But that takes longer than making another copy.)
Maybe if I did a "diff" between raw and played worldgen saves I'd find that it'd actually be easier to 'revert', if there's a file for the player's delta (and if player-led historic events get irremovably integrated into the world history store, I could live with that if it's just a reversion to the original geography that I'm concerned about). But I've never actually 'hacked' a save, merely given myself the aforementioned savescum option.