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the old Thief2 demo has been restored to a working condition - it can be downloaded here (links also added to the sticky).
I tried to download it, but it seems impossible now. I did a search and found another link instead for those who are interested:

Just started it up, and it works on Win10 without problems!

The level is in parts different (and in my opinion superiour) to the original level in the game! Have fun!