I can state the reason I stopped hosting my exit relay was due to tor rpm package not being up to date for CentOS 7. The last available version was considered out of date and no longer supported. So instead of running a relay that was potentially detrimental to the health of the tor network I shutdown the node.
On Oct 23, 2017, at 9:32 AM, David Goulet <dgoulet@torproject.org> wrote:
Hello everyone!

Since July 2017, there has been a steady decline in relays from ~7k to now
~6.5k. This is a bit unusual that is we don't see often such a steady behavior
of relays going offline (at least that I can remember...).

It could certainly be something normal here. However, we shouldn't rule out a
bug in tor as well. The steadyness of the decline makes me a bit more worried
than usual.

You can see the decline has started around July 2017:

https://metrics.torproject.org/networksize.html?start=2017-06-01&end=2017-10-23

What happened around July in terms of tor release:

2017-06-08 09:35:17 -0400 802d30d9b7  (tag: tor-0.3.0.8)
2017-06-08 09:47:44 -0400 e14006a545  (tag: tor-0.2.5.14)
2017-06-08 09:47:58 -0400 aa89500225  (tag: tor-0.2.9.11)
2017-06-08 09:55:28 -0400 f833164576  (tag: tor-0.2.4.29)
2017-06-08 09:55:58 -0400 21a9e5371d  (tag: tor-0.2.6.12)
2017-06-08 09:56:15 -0400 3db01d3b56  (tag: tor-0.2.7.8)
2017-06-08 09:58:36 -0400 64ac28ef5d  (tag: tor-0.2.8.14)
2017-06-08 10:15:41 -0400 dc47d936d4  (tag: tor-0.3.1.3-alpha)
...
2017-06-29 16:56:13 -0400 fab91a290d  (tag: tor-0.3.1.4-alpha)
2017-06-29 17:03:23 -0400 22b3bf094e  (tag: tor-0.3.0.9)
...
2017-08-01 11:33:36 -0400 83389502ee  (tag: tor-0.3.1.5-alpha)
2017-08-02 11:50:57 -0400 c33db290a9  (tag: tor-0.3.0.10)

Note that on August 1st 2017, 0.2.4, 0.2.6 and 0.2.7 went end of life.

That being said, I don't have an easy way to list which relays went offline
during the decline (since July basically) to see if a common pattern emerges.

So few things. First, if anyone on this list noticed that their relay went off
the consensus while still having tor running, it is a good time to inform this
thread :).

Second, anyone could have an idea of what possibly is going on that is have
one or more theories. Even better, if you have some tooling to try to list
which relays went offline, that would be _awesome_.

Third, knowing what was the state of packaging in Debian/Redhat/Ubuntu/...
around July could be useful. What if a package in distro X is broken and the
update have been killing the relays? Or something like that...

Last, looking at the dirauth would be a good idea. Basically, when did the
majority switched to 030 and then 031. Starting in July, what was the state of
the dirauth version?

Any help is very welcome! Again, this decline could be from natural cause but
for now I just don't want to rule out an issue in tor or packaging.

Cheers!
David