@Bunsen, this is the general posting about networking and pppd. I mean who has made a connection with an old Mac to a Pi. What serial cable are you usin?. For Mac to PC I have build my own. Is the a standard Mac to Pi serial cable available?
I'm confused by your question. Perhaps we are having language difficulties?
No, there is no "standard" Mac to Pi cable, just as Mac serial ports are not "standard", but we are accustomed to having to find or make adapters for them.
My point was that a USB-RS232 converter is a
lot easier to find and a
lot cheaper ($5 or less on ebay) than a Localtalk-Ethernet converter, and there is no speed difference. There may be some out there that end in miniDIN-8, but I haven't seen them. I have seen them ending in bare PCB holes or pin headers: one could solder a Mac miniDIN-8 lead onto there.
A USB to serial converter, like that used in the guide I linked, will work exactly the same whether the USB end is connected to a Pi or any other *nix computer. In that sense, USB to RS232 is "standard", sort of, and the Mac's Localtalk ports are RS232 compatible; one would make the connection via adapters, or a self-made cable like you did. There are also USB-RS422 converters, if anyone felt that was necessary.
There is also a serial port coming from the Pi's GPIO which one could use as an alternative. This port is at TTL levels, which would require a MAX232 or similar RS232 level shifter chip. This is an easy (beginner's level even) hardware project documented all over the electronics hobbyist web, is no doubt available from a dozen different vendors as a ready-made shield or HAT, and should work identically to any other serial port on a *nix computer.
If someone wanted to get really fancy they could make up a cable with miniDIN-8 on one end and Pi GPIO connector on the other and a MAX232 hidden in the headshell.
Which approach is preferable is a decision for the user.
tl;dr: serial is serial what's the problem?