Not a "myth".
It used to be true, just not so much anymore, as I alluded to in the statement.
I will still choose to do so on mine. I'm not willing to set my battery on a concrete floor for a month if I'm working on my truck, only to find out that it's dead when I'm ready to go back together.
In would not set a battery out for a month without putting it on a trickle charger. Especially an agm.