@feld @wictory @incin obviously not, but I remember using it relatively recently to give some box IP connectivity over the serial cable. But fortunately even the ancient Sun boxes can do PPP.
The last time I set up slip commercially for Internet access was around 1997 with MTU of 1008 bytes. Had to email some large site admins to stop filtering ICMP to make Path MTU discovery work on their Suns.
#freebsd became a SLIP router replacing KA9Q :)