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According to the dev Michal Brzozowski:
"I think Steam no longer supports older Windows versions, that's why I put 10 as the minimum requirement. If KeeperRL ran fine on your Win 8 machine then it should continue to do so. "

Source (steam forums):

Then, a couple of users state that they were able to run KeeperRL on win7 and win8. One of them was using a GOG version. However, they had some issues with the starting loading time being too long and/or the game being unresponsive during the initial load.

I think this information is important as many GOG users are fans of old gaming and tend to have computers with old software. I wonder if more GOG games stated as "Minimum Requirements: Win 10" could also be run on older versions of Windows.
Good catch Hollyhock :-)

I'm guessing that considering Microsoft doesn't consider anything before Windows 10 supported, or safe to go online with, people assume things won't run either.
I can confirm that KeeperRL works just fine running Win7 on hardware from about 2010/11. Regarding load times, if you keep completely offline (ie. don't load saved dungeons, etc.), loading is very fast and I haven't had a single issue thus far with saving or anything at all. Just my experience...
I hope that this helps!