Communicating that there will be unsolvable puzzles is easy. My ealier example was enough for my players to understand that without me saying anything. I was asking for the blood of a race that doesn't exist any more. They went around asking about that race, discovered it was extinct. They went around trying to find a (conserved) blood sample, they couldn't find, obviously. They realized it was an immersion tool, not a mechanical challenge.

Communicating that there will be unsolvable puzzles is easy. My earlier example was enough for my players to understand that without me saying anything. I was asking for the blood of a race that doesn't exist any more. They went around asking about that race, discovered it was extinct. They went around trying to find a (conserved) blood sample, they couldn't find, obviously. They realized it was an immersion tool, not a mechanical challenge.

#Out of Character solutions

#In Character Solutions

##Note on how the challenge was presented

Out of Character solutions

In Character Solutions

Note on how the challenge was presented

First, I'll say that I'm not answering your title question. "Fairness" is opinion-based. I will, rather, answer the body question - how to communicate that a puzzle is (currently or permanently) impossible. Also as a side note, I was a DM very worried about "fairness". I'm now more worried about being fun than fair. It was clear that your approach was not fun, neither for you or for your players.

For similar situations, I have had puzzles that were simply not solvable. I'm also a blood-loving person, so, as an example, there was a puzzle in a ruin from old civilizations that required blood from a race that was long (>200 years) extinct. Without Wish, AFAIK, that was impossible. In my case, that made sense for me - they were exploring something made ages ago, time passed, things changed. The puzzle was not impossible when it was made - it is now, and it will probably be forever from now on.

For similar situations, I have had puzzles that were simply not solvable. I'm also a blood-loving person, so, as an example, there was a puzzle in a ruin from old civilizations that required blood from a race that was long (>200 years) extinct. Without Wish, AFAIK, that was impossible. In my case, that made sense for me - they were exploring something made ages ago, time passed, things changed. The puzzle was not impossible when it was made - it is now, and it will probably be forever from now on.