So instead of "the first domain name was registered", maybe it would be more accurate to say "the first registration of a domain name with a registry took place"?

Basically it was the first ".com" domain.

If anyone knows: How were RFCs distributed before the modern Internet as we know it existed?

In the very early days (from RFC 3):

  DISTRIBUTION

  One copy only will be sent from the author's site to"

        1.  Bob Kahn, BB&N
        2.  Larry Roberts, ARPA
        3.  Steve Carr, UCLA
        4.  Jeff Rulifson, UTAH
        5.  Ron Stoughton, UCSB
        6.  Steve Crocker, UCLA

  Reproduction if desired may be handled locally.

https://www.rfc-editor.org/rfc/rfc3.html

Before the Internet there was Usenet and a world-wide network network of machines connected together via UUCP, doing email and such over point-to-point dialup (modem) links. Sending mail was a matter of knowing a complete path through the network from your source machine to the recipient's destination machine (and Usenet news implemented public discussion groups over this network). If I recall correctly, you could send specially formatted email messages to the server where the RFCs lived, and an autoresponder program would read your message and email back the requested RFCs.

That was still the Internet, it just wasn't the World Wide Web or modern (SMTP/POP3/IMAP) email.

No, that was a network, but it was very much not the Internet. Different technologies.

Per Wikipedia, "The Internet (portmanteau of interconnected network) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to link devices worldwide."

The network of UUCP-connected machines (also called UUCPNET) did not use TCP/IP and had no inbuilt notion of how to get packets from one place to another. One had to specifically list a set of instructions, a path, from source to destination hosts. UUCPNET was entirely store-and-forward, and sent entire messages, not packets. And it was in wide use throughout the 80's before the Internet became a thing (which happened once researchers started interconnecting networks that used TCP/IP).

To say that UUCPNET was the Internet just without the web, is like saying that radio _is_ TV, just without the pictures.

This is quickly going to devolve into semantics, but just because the Internet today is built on TCP/IP doesn't mean that's a requirement to be considered "internet". An internet is just a WAN between LANs, and I don't think it matters what specific Layer 3/4 protocol you choose. ARPANET is widely acknowledged as the predecessor of what we today call "the Internet" and it predates TCP/IP.

Then fidonet and every other bbs network was also the internet. No.

I believe these were distributed via FTP from InterNIC at SRI[1].

[1] https://en.m.wikipedia.org/wiki/InterNIC