Polygonal rifling and lead generally do not go together well, so no it isn't a myth that lead bullets are not recommended in Glocks.
Edit: I have seem some posts by people who have obtained conventionally rifled drop-in barrels for shooting reloads in Glocks. (Storm Lake? Not sure.)
Yes, polygonal barrels and lead are not a good thing.
I have drop-in Lone Wolf Barrels for my 19 and 36 that have traditional rifling. They work just fine. I think the address is lonewolfdist.com