So what I see if RFC 1738 (URL spec) defines "file" along with a bunch
others.

It is listed as "obsolete", but what obsoletes it?  There are two RFCs
listed:
RFC 4248 - Telnet URI scheme
RFC 4266 - Gopher URI scheme

I believe the intent there is that those URI specs obsolete the respective
text related only to those URI schemes, not to the whole document.

There is also other text that "updates" RFC 1738, including:
RFC 1808 - Relative URLs
RFC 2368 - Mailto URL scheme (obsoleted by RFC 6068)
RFC 2396 - URI Generic Syntax (obsoleted by RFC 3986)
RFC 3986 - URI Generic Syntax (updated by RFC 6874 - IPv6 stuff)
RFC 6196 - Making mailserver URI scheme historic
RFC 6270 - The tn3270 URI scheme

So, how can an RFC be "obsolete" by RFC 4266, for example, but then later by
"updated by" RFC 6270?  I think the answer is that RFC 1738 is not actually
obsolete, but just those parts for which new documents were created are
obsolete.

The odd thing, though, is that RFC 3986 still makes explicit reference to
the "file" URI scheme (e.g., Section 1.1).  That document definitely
"updates" RFC 17838.

But it makes me wonder... which of the other URI schemes were not "updated"
and probably should be?  These are in that spec:

   ftp              File Transfer protocol
   http             Hypertext Transfer Protocol
   gopher           The Gopher protocol
   mailto           Electronic mail address
   news             USENET news
   nntp             USENET news using NNTP access
   telnet           Reference to interactive sessions
   wais             Wide Area Information Servers
   file             Host-specific file names
   prospero         Prospero Directory Service

These are reserved:

   afs              Andrew File System global file names.
   mid              Message identifiers for electronic mail.
   cid              Content identifiers for MIME body parts.
   nfs              Network File System (NFS) file names.
   tn3270           Interactive 3270 emulation sessions.
   mailserver       Access to data available from mail servers.
   z39.50           Access to ANSI Z39.50 services.

Of all of these, only mailto, telnet, tn3270, and mailserver URI schemes
have new documents published?  Seems like "file" is not alone in appearing
somewhat abandoned.  I think the truth is that it's not abandoned, but the
documents make it appear that way ... I don't think that was the intent.