A bit complicated but... I'm playing around with the json files, decoding it all, etc. And that was left open by the devs purposefully, to allow players to monkey with their own games, which is great!

However, this allows people who know how, to unlock Steam Achievements. I unlocked one accidentally when testing my edits to some json files, and felt bad about it. Perhaps all Steam achievements can be unlocked in one go simply by loading up a save file someone's given them. Which is probably a bad thing.

I thought of a way you can still allow messing with the save files, but prevent monkeyed-with save files from unlocking Steam Achievements, in a performant way. Internally track the modified date when saving the save files from the game. Then if a game is loaded and any of its files don't have that exact modified date, block any achievements from coming from that save slot.

Regular players will never be blocked this way. Players messing with save files won't be locked from doing so. And unscrupulous achievement hunters won't be able to insta-unlock all achievements.