@gt After looking at the system files once again, I figured out how the menu bar is organized. The indicators request a preferred position in the bar. For the date and time this preferred position is 20 for example. The date and time appear on the far right of the bar. The message indicator has a preferred position of 100, so it appears on the left side of the bar.
In an ideal fix, this entire system would have to be rewritten, with some fixed indicators appearing to the right of the notch and the rest placed to the left. But now the bar is indeed dynamically laid out and updated from the right side of the screen, causing some indicators to disappear behind the notch.
It's possible to change the position of indicators. You could temporarily give the messages indicator a preferred position of 22. The envelope will then appear to the left of the date and time in the bar.
For this you need to edit the file "com.canonical.indicator.messages". This can be found in "/usr/share/unity/indicators/".
Note the selected line "Position = 22". Don't forget to restart Lomiri after the adjustment.
This also is a hacky solution, but hopefully it will solve temporary frustration, until the notch problem is properly and universally addressed.
@gt for uMastonauts, @Krille needs to move the app to QtWebEngine but it seems he's having some trouble with that, and for Duolingo the xenial version has not been tested yet as you can see in https://open-store.io/app/duolingo.aitzol76 (I'll ping him in the Spanish group).