Not really
deadly compared to other Amanitas, like the Destroying Angel or the Death Cap - those are truly lethal even in small amounts.
Years ago, an eccentric old mycology professor here made her students eat sauteed Fly Agarics during the summer fieldwork as a sort of initiation ceremony. The toxicity is also said to depend on factors such as the weather, regional differences, and individual genetics. Some collections cause no ill effect whatsoever, and people say they actually taste nice when cooked, some cause very unpleasant gastrointestinal symptoms and sweating from the muscarine... plus some people eat the Fly Agaric to get high (it's said to induce deep sleep with vivid dreams and hallucinations). I am not aware of any successful attempts to achieve that state among the few substance abusers who I know personally, and a couple of dopeheads who did claim there was some effect probably had their blood so full of other types of junk that it may well have been something else, not the mushrooms.
Some books claim that decades ago, reindeer herders here in Western Siberia used to eat Fly Agarics to get high, and their reindeer were said to eat the snow where they peed because most of the hallucinogen is excreted by the kidneys unchanged.
I'm not sure that it was really a popular recreational or ceremonial drug though, because the Khanty and Nenets people switched to vodka quickly and permanently when it became available. I know a Khanty elder who actually performs plenty of ceremonies (for instance, he helped a friend's family find the body of her brother-in-law, who had drowned), and he has never used Fly Agarics. I know of just one Khanty guy who tried eating Fly Agarics to "connect with the spirits", however, all he achieved was a nickname, "Mukhomor" (meaning Fly Agaric), that has stuck for years. The guy isn't very smart, he shot two redlisted reindeer right in the snowmachine tracks of the nature reserve rangers who were out patrolling the territory. It didn't take long to catch him. The judge pitied him and he didn't go to prison (he did, later, when he shot another guy in a drunk altercation)....
