> Do we even know if smallpox would kill a modern human?

The last person to die of smallpox died in my lifetime. I hope I'm not so old that I'm no longer considered a "modern human".

At least in countries like the USA there is still some herd immunity. Everyone over the age of 40 was vaccinated and also some people younger than 40.

That "age of 40" number is years out of date. Routine smallpox vaccination for children in the US stopped after 1971, after the disease was declared eradicated in the Americas. The vaccine was given at age 1, so people born in 1971 generally did not get it. So to get to "everyone" you have to look at people aged 47 or above.

Not really. First I'm over 40, and was never vaccinated.

Plus, effective heard immunity requires something north of 85% of the population. Smallpox vaccination rates are not even 50%.

Military personnel are still vaccinated.

Do we know if the vaccine works against modern “weaponised” strains of smallpox?

Vaccination would presumably be adjusted for by anyone weaponizing small pox, or any other biological wmd for that matter.