Hiya!
As some of you may know here on the forums, I am what I would probably call a "Killer DM" (for nowadays anyway; back in 1e I didn't have this title...well, not much anyway...

). Ahem. Anyhoo...I think it all boils down to if the Player(s) felt that it was a more or less 'unknowable surprise' or not.
My players, on occasion, give me a bit of "contention" when a PC is struck down by some singular die roll. When I tell them why, and what they ignored, they usually sigh, flagellate themselves with a wet PHB, and make a mental note to pay more attention to the game.
As long as you, the DM, have done your job in the Foreshadowing department...you should be golden. Even if the PC's didn't discover whatever that/those clue(s) were...as long as you can show them or point it out to them after the game, they should understand.
For example, if they are in an old mine infested with goblins. They're kicking ass and taking names, make a turn down another corridor, and you suddenly spring a "Dex Save, DC20 as the floor opens up under you, a lake of lava 30 feet under you! You fail, you die!". That would be OK or Not Ok, depending on if you have put a means to discover that the old mines were abandoned because the workers discovered what they were mining down to. An old journel mentioning it, an old sign with "STOP! Lava below! NO MINING!" flipped over in some chamber, and/or some NPC/Monster that tried to say "Wait! I have secrets!" before the raging barbarian chopped his head off. If the Players don't bother to see what 'that old book' was, didn't flip over the old upside-down sign in the corner of some chamber, and killed a goblin who would have told them the 'secret' of the mine...that's on the Players. Not you. Totally fine to have the "Save or die" thing. If, however, you didn't have any of that...well, how would the Players/PC's have any inkling that there was an active lava zone 30' below them? In that case, yeah, they should yell at you. I mean, the miners who discovered the lava would likely have put up signs, rope, etc warning folk about it lest any brash kids/teens decide to try and play there; basically, an "Enter at your own risk" thing. It would be logical for these sorts of clues to be available for discovery by the PC's. If you fail to include these logical conclusions, well, that would be on you.
It's all about fairness. If the Players see a Save or Die as "unfair", even after you explain (after the game/adventure, obviously!) that they simply ignored or missed every clue...well, not much you can do about that. Most players, I would hope, would understand.
So...feel free (hell, I
encourage you) to put in "save or die" traps and situations every now and then just to keep up the premise that 'adventuring' is a high risk / high reward "job". Just remember to
Foreshadow in a couple places. If the Players miss/ignore these clues...it's all on them. It's not your job as a DM to help the PC's succeed. Always remember that.
^_^
Paul L. Ming