I've noticed that my SSH sessions are timing out, not before 5 minutes of inactivity. I looked in my ssh config file, and didn't see a timeout set. I didn't have this issue before. Here are two of my iptables rules, which is I believe, to be the culprit somehow:
I believe that I am missing an additional rule, as possibly packets are being dropped, closing out my session. Oddly enough, nothing shows up in my auth.log file when the session is terminated.

There are many things that can enforce a timeout.

Within BASH itself the TMOUT variable can be set e.g. in /etc/bashrc or $HOME/bash_profile or some other environmental file.

A firewall device (e.g. Cisco ASA or Sonicwall) could set a timeout.

Here we actually do both of those but there are other things that might do it. (e.g. A cron job.)

My hardware has not changed.
I could not locate either of those files. I am running Ubuntu Server 16.

Show the entire iptables rules. iptables -vnL INPUT and OUTPUT.
How does the session terminate? Does it say broken pipe? Does it happen suddenly, is there a timeout before you get an error?

Those would be the only two rules related to ssh. Does a forwarding rule need to be put in place? Is there nothing wrong with these two rules?
The session just closes unexpectedly, with no error or timeout.

Well, you're quite reluctant to share information for someone who actually needs help. On the other hand, you're asking if ssh access needs a forwarding rule, which leads me to believe that you don't understand iptables (and that's ok), but you're not actually showing us what is going on there (which isn't ok). Moreoever, you haven't actually shown us what sshd_config looks like, you're just saying there's no timeout directive. What would a 'timeout' directive look like in ssh exactly? Are you referring to the correct two directives? Also, it wouldn't hurt to know what distro you're using...
EDIT: Sorry, you've already said what OS you're using. Are you using it as an independent bare metal server, or as a virtual machine/container?