It's not a complete myth, there certainly are case reports of pyrethroid insecticides being used as a stimulant. Only one study that is available on google, however this report mentions and refers to several other reports of pyrethroid poisoning via intentional use, which I assume are not easily found or accessible by the public.
This is a case of a 22-year-old Hispanic male with a history of bipolar disorder and methamphetamine dependence who was admitted after presenting with suicidal ideations by slashing his throat with a machete. The patient had been smoking and ...
www.ncbi.nlm.nih.gov
Basically, pyrethroids poison you, but if ingested quickly (IV/smoke/snort), it does provide a "rush" similar to methamphetamine... before going on to poison you and damage all your organs.
Probably only done by people in psychosis, or extremely disturbed people, who are addicted to meth/stims.
It's like strychnine, which was once touted for it's stimulant and sports performance enhancing abilities.... but anything above a tiny dose is a deadly poison that kills you.
But yeah, reporting on this is just a scare story. People aren't doing it large numbers. Like a druggy urban legend but does have 1% of truth behind it.