Myth. I sometimes sit and dry fire my pistols for hours while watching tv. It helps break in the trigger and also helps you get better at pulling that trigger. You can learn to remember the trigger so its never snatched and what not.
I’ve heard of people that will dry fire 1911s 500 times before even shooting it to break it in a little before taking it out. It gets a lot of the “shittyness” out of the trigger and even the slide mechanisms
Old guns it could be bad, but that’s what snap caps are for. I wouldnt do it to a old break action gun or anything but the modern pistol or rifle its fine.
I also dry fire any gun when I’m handling it at a shop. Gun may feel great but you need to like the trigger before buying it.