I've now figured out how this can be done, but it's unnecessarily difficult and not intuitive because the Unread column is all the way to the right of screen out of box and doesn't show even on my wide monitor without scrolling. I can of course RTFM, but I don't want to have to do logic just to toggle read/unread in an ~inbox. Anybody reading here's make-it-work:

1) Make sure you have the Unread Column showing in Messages
2) Move the Unread column to far left to make it easy to click and toggle filter in future
3) Click the funnel icon in Unread column header
True = Unread
False = Read

Here's my case for making Read status a toggle in Ribbon:

1) Make-work as stands means you have to be familiar with concept of filtering data... transferrable skill from Excel, but not a skill reasonable to assume for the sample set of all people who will need to check read/unread status... I guess you could argue myStratus users would be self-selecting and able to cross that threshold, and that would be true about me personally but not about the people I'll be training to read messages for me.

2) You have to do logic in your head to go from True->Unread when making the filter; Showing Read you have to compute a double negative: Unread.False = Read(.True).

3) "Blanks" should be removed as not helpful to logic. It's like saying is a number something besides positive or negative? There is only one answer... 0. Corollary here: is there a text not read or unread (a boolean)? Answer is... no, nothing fits that, so I'll show you a blank screen if you choose that. It's just clutter, like on multiple choice where A) Right Answer, B) Wrong Answer, C) Red Herring, D) All of Above