Yep definitely a myth.
That said I'll happily make them for anyone, I won't fit them for people, that's their responsibility and their head on the chopping block.
My car pops flames with the metal cat for sure, but they were alot bigger with the old decat fitted.
Plus it does make a bit of difference to the note/sound of your car, the 2 lads at work with s14s def sound alot more tractor spec with the decat fitted.