I've never heard of not attacking a PC because they were squishy. If that were yhe case then they would nees HP or defenses. Might as well say they are immune to all attacks until they reach a certain level.
Not immune to attacks, it's just that in 1e/2e (and to a lesser extent 3e), there was nothing really protecting the Wizard at all. ESPECIALLY at first level. So, any enemy that went out of their way to attack the Wizard would pretty much automatically succeed.
So, I know that in our games(and I assume in many others), the first couple of games went like this: "The enemies go, they run around the games in the front line to kill that annoying Wizard in the back. They hit, they do 10 damage....uhh, the Wizard dies." Generally in the first battle the Wizard was ever in. After the first Wizard died, the second Wizard would show up and die in their first encounter as well.
Many DMs realized that killing the Wizard wasn't exactly difficult to do. It wasn't a great feat...in fact, if you didn't go out of your way to AVOID killing them, they died without you trying.
5e isn't quite as bad, they can take a hit or two. However, so far nearly all of the PC deaths in my games have been Wizards.
At this point, I still naturally look for excuses not to attack the Wizard. Most of the time, I rationalize like crazy:
"Well, the Wizard is further back, do I want to run through the front line and be surrounded?"
"That guy with a sword is closer, it requires so much more walking to get to the Wizard...and that sword looks pretty dangerous."
"The huge guy with the muscles looks extremely dangerous, that guy in the robe looks like a weakling. It isn't very honourable to fight weaklings. I'll finish him off when I'm done with the worthy enemies."
"That guy in the robes could be a Wizard, but maybe not. He hasn't cast any spells yet. Maybe he's just some unarmed peasant who likes robes."
"That wizard cast a cantrip that did 3 points of damage last turn while that Fighter did 10 damage with his sword and killed one of my friends. The Fighter is the greater threat and needs to be taken out now!"
"This is a Wolf, he doesn't know what a Wizard is. He attacks the nearest thing."
It has the net effect of having the Wizard not be attacked most battles. Really, most of my enemies attack the nearest person. If the Wizard runs up past the Fighter he'll still get attacked because most of my reasons melt away then. But Wizards playing defensively in my games will often not get attacked during combat.