Back in 2000 my first laptop had a serial port and i bought a Wacom serial based tablet for it. A couple of years later that was gone and i had to buy a USB Wacom (that still works with win10 on modern machines!!). So for some other HW project i had to find a USB-Serial converter, so the question will be, are we really supporting the same old standards if we're forced to chase dongles?
Mac OS still ships with BinHex (https://en.m.wikipedia.org/wiki/BinHex), I think, so for Macs, that might already be the simplest way to exchange data between an early Mac and a modern machine (even if you have a floppy drive, does it read Mac disks?)
Edit: maybe my memory is of, and it was direct dial-up to another machine, not across the internet. NCSA Telnet (https://en.wikipedia.org/wiki/NCSA_Telnet) supported TCP/IP before the OS did (yes, that was possible. I don’t know whether you still could do that on a modern system), though, so who knows?
In general you can. If the OS lets you open raw sockets, you can implement TCP and all yourself in userspace. Although in those days, I'm guessing it was actually implementing the entire AT modem stack too, and just driving the modem directly via a serial port.
I'm sure you could. In 1985-ish.
How much of the required hardware still exists? How much of the required software can you find? Is that software (or another version of it) also compatible with your modern computer? etc.
I saved my work as a text file on a USB stick, which was no problem bringing over to my modern Mac.