The feeling that you’re suffocating is triggered by CO2 buildup. Helium is the suicide gas of choice but nitrogen does the same thing. It displaces the CO2 in the air so there isn’t any build-up.
If you’ve ever breathed from a helium balloon and it made you light-headed, continuing to breathe it would have made you pass out. Breathe it for long enough once you’ve passed out and you die.
I’ve never passed out but I’ve breathed it enough (so I could talk like a chipmunk) that I know it isn’t unpleasant.
There’s also sulfur hexafluoride, which has the opposite effect on your voice, making it sound lower-pitched.