Argyle King
Legend
To give the absurd form of the argument:
If the GM puts Tiamat on the first level of the dungeon and allows 1st level characters to walk into it without warning, their death pretty much is on the GM's shoulders.
Now, maybe you personally are so fantastically good that you never over-challenge without telegraphing it. But as a generalization, GMs cannot (and should not) outright disavow culpability. The GM is part of the process, and some of the responsibility can thus fall on them.
I would not say I necessarily disagree with this; however, it assumes a game where things such as level and CR (or XP budget) are built into the game. That is not the case for all games. There are plenty of rpgs which do not have levels.
If I'm running a game where there is no such thing as 'encounter design' in the same way D&D typically uses the words, and I am instead running a game in which the world evolves naturally and responds to the players, I do not believe it is my fault if the PCs willingly seek out a being such as Tiamat and find hardship & death. I am not suggesting the players in my games never encounter anything unexpected. However, I am suggesting that I prefer to run a game in which things evolve in a manner which is tied more to the in-game world than tied to out-of-game concepts such as level.