Startrail: Is there any thread that lists out whether a demo contains unique elements ....
Not specifically that I am aware of, though maybe some gamesite listing somewhere.
Someone may mention such in passing in this thread, or more likely in the forum section for that particular game.
The most likely place though would be the developer website. If it has been intentionally made different, then surely they would mention it, and they certainly do sometimes here on the game's sale page.
Changes that occur during development would be more problematic, and unless there is a good reason to mention it, they probably don't want to. Hell, we even had a demo removed from GOG a few months back, because the full game was now substantially different. GOG even had the cheek to remove it from our libraries, which is just not on. I see it akin to attempting to rewrite history. Our libraries should not be tampered with unless it is truly necessary, and a demo doesn't qualify unless it is harmful to our system or contains something illegal.