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If the PCs use magic to gain entry into an unknown space without the ability to magic their way back out, that's idiotic for any number of reasons. Whether the atmosphere is toxic is almost besides the point.
You do something THAT unwise, it's hardly "rocks fall, you die" territory. It's like complaining the GM isn't rolling dice for the damage dealt when you stick your head into a guillotine trap. "Player, May I?" only gives the player so much agency. The GM is still the GM, and when you do something that you shouldn't live through, you shouldn't. Even if you "had no idea what would happen". That almost makes it even worse.
If you didn't know the possible hazards, then what were you doing messing with them in the first place? Don't go sticking your head into holes in the wall, and for Gods' sake don't teleport blindly into a place you know you can't teleport back out of.