What was common was the SLIP line discipline code under the covers, but how the line got set up to start running it was different.  The original SLIP 'kit' from Harvard/MIT lacked anything like PPPD to start and was sort of ad hoc. I think originally it used a program called 'chat' that had been pulled out uucico that set things up then exec'ed the SLIP stuff; but chat(1) may have been round 2 or 3.   I've forgotten the name of the original daemon, I bet it you look in the Usenet archives from 82-84 and look for 'SLIP' you find a couple of things.