nowadays it is usually not the default anymore, since tcp connections and unix pipes have other ways to avoid overflowing their buffers. the history is that that keybinding originated on its. as i understand it, its also had another flow control protocol to avoid overflowing serial terminal buffers, the moral equivalent of the enq/ack scheme, which doesn't depend on hard real-time responsiveness to work reliably. (but it wasn't just literally ^e and ^f since emacs uses those even more than ^s.)
initially to get emacs running on unix systems you had to do a lot more than just stty -ixon -ixoff; you had to upgrade your host's ram and convince the other users that an editor was a reasonable use of such a large amount of resources. by comparison, adding a couple of extra wires to the modem cable to support rts/cts flow control (which was more reliable anyway) was no big deal
but yeah, it was a big pain for about 15 years, from 01990 to 02005. it still bites me occasionally when i accidentally type ^a^f in screen and accidentally enable xoff!
initially to get emacs running on unix systems you had to do a lot more than just stty -ixon -ixoff; you had to upgrade your host's ram and convince the other users that an editor was a reasonable use of such a large amount of resources. by comparison, adding a couple of extra wires to the modem cable to support rts/cts flow control (which was more reliable anyway) was no big deal
but yeah, it was a big pain for about 15 years, from 01990 to 02005. it still bites me occasionally when i accidentally type ^a^f in screen and accidentally enable xoff!