Sort of....   As the first customer for UNET I can verify that there was a serial interface that had been used to for debugging the SW until the 3C100 became stable.   But it was not what we would later call SLIP.  It was really not usable for much other than to debug the protocol.  It did not have a 'chatting' and was for dedicated (hardwired) serial lines.  As I remember it, the code mostly was used as a loopback between two TTY ports and IIRC it only worked on the DH11.   I think Greg Shaw wrote it, but it might have been Bruce Borden (I can ask Bruce).   Glaser and I set it up in lookback mode as we used it to A/B test the Hyperchannel code we wrote.

My first experience with a real 'SLIP' as we later knew it was a distribution from Harvard/MIT in early the 1980s (83/84 IIRC but dates could be off).  The code base took some major rework to the TTY driver, that I thought had originated at MIT.  IIRC I got the sources from someone like Jack Test or somebody else working in Steve Ward's real-time lab (i.e. Terry Hayes/tjt may have brought the src with them); but we could have just has likely gotten them sob @ harvard.   It ran on DH's as DZ'd were an issue because of interrupts as I remember (DZ's always were an issue when you pushed them).  But, it must have originally worked on BSD 4.1 not 4/1C/4.2, which tells me it was hack off the original BBN code which did not use sockets, but rather the same open("/dev/tcp", ...) stuff that chaosnet used.   But the times are fuzzy in my mind, so it's possible by the time we got it, worked with the UCB code sockets code base.   I do remember that it ran on sockets by the time Steve Zimmerman hacked SLIP into RTU when he redid our TTY driver.

The important thing is that the early version lacked PPPD, as we were later know it.

There were two or three different PPP schemes in the beginning.  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.     The more modern pppd(1) coming from CMU is as likely as anywhere else.   I did eventually run it at home on a FreeBSD box, but it was at least the second or third way I set up my dial-ip ISP connection.