You can buy books on how to make and obtain chemicals on your own.

Hell here's an Internet Archive book on making explosives

https://archive.org/details/saxon-kurt.-fireworks-explosives....

If you ever chat with older folks pre-90's much of this information was accessible fairly easily. It only changed with the push by the government to crackdown on Waco, Oklahoma City bombing, militias and other related groups. There was then a campaign to make it "normal" to limit free speech on the subjects, where as these books were available before.

I think the whole thing where AI should make information less available is a difficult battle and one which I personally oppose, but do understand. Free speech and information isn't the problem, it's the people, actions and substances they create.

After the age of the internet, I think it's been a forever loosing battle to limit information, it's why we couldn't stop cryptography, nuclear weapon proliferation, gun distribution, drug distribution, etc. The AI is just another battle ground, one which, if they actually do manage to control could definitely create some walls to this information, but not stop it.

More scary, is that the AI as it becomes pervasive and stop people from asking certain questions, because they don't know they should ask... but that's unrelated to the risk of mass death.