I'm in the eu. I can legally get my hands on research benzos. Research opioids are a bit harder for me to source, but probably not impossible.
If used some RCs either recreationally or as form of self-medication in the past.
And I've seen a friend with an accidental OD. He did black out and survived even without medical intervention.
So I'm 100% sure research benzos work for inducing sleep, anxiety relief and so on. I consider them as perfectly valid alternative to prescription benzos for the purpose of blacking out when combined with another method like SN.
However, as with all benzos it seems nearly impossible to die from them alone.
Some threads here mentioned RC opioids. While harder to source, to me it is at least plausible that a well informed combination could mimick the mode of action of N.
That being said, I'm not an expert in pharmacology and information about these substances is extremely rare and scientifically documented case reports are lacking.
So I remain skeptical but slightly optimistic. We should really try to collect as much research as possible.