Devuan aims only to be "Debian with no systemd" though it's not, and never going to be, that simple.

I would be a little saddened but not at all surprised if some version of of Debians X and Architecture plans found its way here as well. 

So you should not be dependant on upstream to decide on your installed package set.

I keep a mental list of a set of packages I need and it grows smaller not larger as time goes by.

In the mail command case there are a few options ... they all work fine with some slight differences. mailx works for me but there are others that are just fine.
update-alternatives  has this at the moment.

/etc/alternatives/mail -> /usr/bin/bsd-mailx

Also exim4 supports all the traditional "sendmail" commands.
including :

sendmail -i 
is actually:

 /usr/sbin/exim4 -i <recipient-address(es)>
         <message content, including all the header lines>
         CTRL-D

       The  -i  option prevents a line containing just a dot from terminating
       the message. Only an end-of-file (generated by typing  CTRL-D  if  the
       input is from a terminal) does so.

see: man 8 sendmail

This also applies to other MTA's postfix for example.

To introduce myself, I have been a Linux user since 2009.  I like to tinker with various distributions, and I also aspire to become a Linux system administrator.  Maybe I will post an extended intro in Off-topic later.

Ok, now to my question:

I wanted to demonstrate Linux to an on-line friend by teaching him to use the basic commands.  My method of demonstration was to create a virtual machine in my home that he could log into remotely and explore that way.  So two weeks ago, I installed Debian stretch, which I assumed represents one of the most typical, predictable, run-of-the-mill Linux systems.  It seems that assumption is rapidly becoming invalid.

Two days ago, I created another VM and installed Debian onto it as well.  I used the exact same choices in the installer, and therefore expected the resulting system to be identical.  Note that I used the Net Installer.

But as I was checking out the newly installed system, I discovered that the 'mail' command was missing.  I was confident that it was not a mistake on my end, so I became determined to know why.  This led to many hours of me trying to figure out what happened to cause this package not to be installed.  Turns out that the MTA, exim4, was also not installed on the new system.  Being aware that the Net Installer relies on the on-line repositories to decide which packages to install, I knew at once that they had somehow changed the 'Std system utilities' package set in the span of two weeks.  But I consider that mailing is an important part of a Linux system.  For one, the cron daemon needs an MTA to mail the output of cron jobs to the user.  There are probably other functions that I don't yet know of that will be broken by this move.  Frustrating.

Long story short, the Debian folks have changed the priority for exim4, mailutils, and several related packages from 'standard' to 'optional'.  Further, after discussing this in various corners of the Debian community, I realized that they aren't likely to undo this change just because cron is affected.  Rather, I suspect that the removal of cron will take place in the future.

I am quite upset.  While I am not upset in the way that a veteran might be, I am upset because I am learning about this thing called GNU/Linux and if Debian keeps changing so dramatically, then it renders what I thought I knew to be inapplicable.  I can't be certain of what to expect from it now.  But come to think about it, this isn't the first time they've done this - they started down this road when they moved to systemd.  I guess I'm just a little slow on the uptake.

I then thought to myself, if these people at Debian feel that having an MTA, or a 'mail' command, or even cron isn't necessary in the default install, and I don't like the change, then perhaps there are other people who won't like the change either.  And then I thought of the folks involved with Devuan.  I thought that since you didn't like the switch to systemd, maybe you will also dislike this new change.

I would like to know if I can continue to have things like the 'mail' command and cron with Devuan's default install.  I suppose many others would want to know as well, seeing as this change only began in the last two weeks.  I have a need for a distribution that is predictable in this way.  If I can get that from Devuan, it would ease my mind.