In our office, we are currently utilizing 2016, 2017, AND 2018 for various projects, (we keep our models in the same version year as the client's model).
If you accidentally open a 2016 project from Revit 2018- as you may know- it automatically upgrades the model to 2018 and the only way to "revert" it back to a 2016-friendly state, would be to restore the .rvt file to an earlier date (and of course risk losing work). There is a cancel button... But there have been times where I hit it too late, and it upgraded anyway...
I noticed in 2018 & 2019 there is a small preview pane which displays the .rvt file you selected and will light up "2016" in the preview. That's awesome! But in the mornings when the caffeine hasn't kicked in yet- it would be very helpful if a little message could yell at me and say "This file was created with an older version of Revit. Are you sure you want to upgrade the model?"