LSD is fairly stimulating as far as classical psychedelics go, and, obviously, extremely potent, it doesn't seem impossible to me that it could kill at obscenely high doses.
The idea that it's simply not possible to overdose on LSD is, I think, a little baseless - and a result of looking at LSD only through rose tinted glasses, so speak.
For one - fatal doses of LSD have actually been measured in animals. The LD50 in mice and rats is known, and while a lethal dose hasn't been definitively proven in humans, or, indeed, any primates, the observed effects of ultra-high doses of LSD are serious enough that it does not seem inconceivable they could cause death especially in more vulnerable individuals, with weakened immune systems, pre-existing conditions or disease, or, probably, if the dose was several times higher.
Source:
https://maps.org/research-archive/w3pb/2008/2008_Passie_23067_1.pdf
Toxicological Data
The LD50 of LSD varies from species to species. The most sensitive species is the rabbit, with an LD50 of 0.3 mg/kg i.v. [52]. The LD50 for rats (16.5 mg/kg i.v.) is much higher [52,53], though mice tolerate doses of 46– 60 mg/kg i.v. [52,54]. These animals expired by paralysis and respiratory failure. Monkeys (Macaca mulatta) have been injected with doses as high as 1 mg/kg i.v. without any lasting somatic effects [55].
There have been no documented human deaths from an LSD overdose. Eight individuals who accidentally consumed a very high dose of LSD intranasally (mistaking it for cocaine) had plasma levels of 1000–7000 μg per 100 mL blood plasma and suffered from comatose states, hyperthermia, vomiting, light gastric bleeding, and respiratory problems. However, all survived with hospital treatment and without residual effects [56]
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Comatose states, hyperthermia, vomiting, light gastric bleeding, and respiratory problems" - so taking the upper limit of survivability - 7mg / 100ml of blood plasma, and going by the average of 5 L of blood volume, plasma making up 58% of that (results of a quick google, someone correct me if I'm wrong but I'm sure it's a good enough ballpark) - we get 7 x 10 x 5 x 0.58 = ~200mg as a dose that's been recorded as survivable, with some possible caveats such as the need for access to a hospital, although it's hard to say if that was really a necessity.
What about if that dose was upped to 100x, say, 20g? Like one of those protein scoopers full of LSD. How about 10 of those? I'd say it's believable this would be fatal. Maybe not, and it's surely very difficult to kill yourself with LSD alone, but surely not impossible.