It predates Linux by a lot. It's software flow control for ancient terminals and modems that couldn't keep up, so they'd use in-band ^S and ^Q characters to man

It predates Linux by a lot. It's software flow control for ancient terminals and modems that couldn't keep up, so they'd use in-band ^S and ^Q characters to manage the input and output. rms also hated it, to the extent that he intentionally used Ctrl-S for the most basic search function of Emacs so that people would have to turn off the ^S/^Q crap throughout the path from the program to their terminal, or they wouldn't be able to use emacs. (Or at least so I've heard; I tried searching for confirmation but failed to find it.)