I've encountered them, and i've seen them in real life. Both my nephews turn into these when they play games. Not online multiplayer, because their parents won't let them.
I've encountered the occasional screamer, but they're mostly not as vocal as people seem to believe.
To all those who say they've encountered one, are you sure it wasn't a squeaker?
I don't know who coined the term, but i've seen it floating around some message boards back when i played CoDBlops intensively.
Squeakers are youtubers, or just people who want some giggles for themselves, who'll modify their voice and complain loudly. There are apparantly a lot of these if you'll look on youtube for 'screaming 12 yo in *game*'.
I've once encountered an absolute horror, squeeky voice, loudly complaining, and at the end of the match, another player announced his channel where you could find a lot of these videos. Always with the same voice and script, in different games. So probably two players working in tandem to create these videos.