The limited-time demo (link for the lazy) has no DRM, so all you need to do to preserve it beyond its expiration date is copy the folder it’s installed to somewhere else. This works with most limited-time demos on Steam.
You can also copy the large number of DRM-free games on the platform to other systems or create backups of them using the same method. Here’s a (likely ver incomplete) list:
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Here is another such list of DRM-free games on Steam: https://www.pcgamingwiki.com/wiki/The_big_list_of_DRM-free_games_on_Steam
Edit: I think this is an outdated version of the list. The following one is more up to date: https://www.pcgamingwiki.com/wiki/List_of_DRM-free_games_on_Steam
