I don’t recall this feature being added till at least cataclysm, I was looking at my map and was attacked by a hostile mob, and the borders of my whole screen started blinking red as if I was about to die and continued to blink until the mob was killed.
Was this in Vanilla or am I just mistaken? I thought it didn’t signal you till cata
Yeah…this happens on pservers as well …vanilla pservers.
Was it always that way? Or do the Pservers get it wrong too?
I don’t remember
If you don’t remember then why are you questioning this ?
Edit: I have addons and one of them does it for my 1.12 client.
So not sure if it was in vanilla wow.
I found something about it being in game in patch 2.3. I guess submit it as a bug report and see what happens?
That’s the reason I’m asking, I’m fairly sure it wasn’t but not entirely certain.
Wow 2.3 Patch Notes: When you take damage and have a full screen UI up, the edges of the screen will flash red so you know you are under attack.
Ahhh okay, cool, thank you. 
Sure thing 
That feature was turned off by default for me when I started my client up for the stress test. I had to turn it on in the options. It’s under combat.
I usually turn it off - it’s kind of annoying. I usually know what my health is and don’t need a bunch of flashing when I’m trying to keep myself alive.