Is it time to kill support for very old servers?

pg_dump alleges support for dumping from servers back to 7.0.  Would v10
be a good time to remove some of that code?  It's getting harder and
harder to even compile those ancient branches, let alone get people to
test against them (cf. 4806f26f9).  My initial thought is to cut support
for pre-7.3 or maybe pre-7.4 servers, as that would allow removal of
support for cases where the server lacks schemas or pg_depend, each of
which requires a fair deal of klugery in pg_dump.

In the same line, maybe we should kill libpq's support for V2 protocol
(which would make the cutoff 7.4).  And maybe the server's support too,
though that wouldn't save very much code.  The argument for cutting this
isn't so much that we would remove lots of code as that we're removing
code that never gets tested, at least not by us.

One small problem with cutting libpq's V2 support is that the server's
report_fork_failure_to_client() function still sends a V2-style message.
We could change that in HEAD, certainly, but we don't really want modern
libpq unable to parse such a message from an older server.  Possibly we
could handle that specific case with a little special-purpose code and
still be able to take out most of fe-protocol2.c.

Thoughts?
        regards, tom lane