There is a difference between "never been shown to" and "shown to never".

Scientists (and logicians) avoid saying "shown to never", as it's much harder - almost impossible - to prove a negative[0]. This is also a black swan[1] type situation. The only way you can say something is never something is if you understand all occurrences of it; otherwise you can only report on what you've been shown.

[0] https://en.wikipedia.org/wiki/Burden_of_proof_(philosophy)#P...

[1] https://en.wikipedia.org/wiki/Black_swan_theory

Yes but the problem with "never been shown" is how it's often used to "take advantage" of people's interpretation like what you list above.

For example dropping a 27.4 kg ball on someone's head has never been shown to cause injury. We've dropped a 26.3 and a 29.5, but we've never studied dropping 27.4 kg - and as a result, "dropping a 27.4 kg ball on someone's head has never been shown to cause injury".

Frequently "never been shown" its used as a weasely way of saying "we've never looked at", or "we don't know if"

>This is also a black swan[1] type situation

So one that is almost certain to happen given some time?

>The only way you can say something is never something is if you understand all occurrences of it;

"Understanding all occurences of it" seems like a good criterion for allowing potentially mass murderous work or not. That said, it sounds stricter than what it is. We just need them to know how a thing works and not be doing "sorcerer's apprentices" style peek and poke work.

They don't need to "prove a negative". For example we don't asks scientists to prove that "water can't explode and destroy the earth" to let them work with it casually. It's enough that they know it well. Do they know prion as well as they do water?

Yes but many experts take "never been shown to" to mean that we ought to act as if it doesn't for now. That's an important and useful social mechanism in the laborious process of constructing a solid edifice of truth from the crooked timber of humanity. But it's unwise when considering public health responses.

You're absolutely right, but it's also true that the phrase "never been shown to" most often functions in a sentence more like "doesn't/can't happen" than "we have no idea."

That was meant to be my point, the down vote makes me think I didn’t make it clearly enough…

"There is no evidence that..." is my recent favorite.

Yeah that’s been constant. There was ‘no evidence that delta variant is more dangerous’ for white a while, until there was actually some evidence to go on.