>SLIP has always been a 296 byte MTU.

"always"?

RFC 1055 declines to be so forceful as to assert a fixed standard size
(it is, after all, a "non-standard") but it documents the Berkeley 1006 byte
MTU and strongly suggests that implementations not send more than that, as
well as being prepared to accept that much.

I seem to recall that Romkey's original PC implementation used 2002 bytes,
though I could be misremembering that.

Two sides of a SLIP line can, of course, be configured for anything you like
(in fact, they HAVE to be, which was a primary motivation behind PPP (the
other major motivation being the desire to transport protocols which IP
hadn't finished exterminating)), but the assertion that "SLIP has always been
a 296 byte MTU" is simply wrong.