With today's release, which brings its version number to 42.0.2.3438, Microsoft Edge for Android gets support for picture-in-picture mode. Although this mobile browser works with Android 4.4 KitKat and later, the aforementioned feature is only available for devices that run at least Android Oreo.
https://www.notebookcheck.net/Microsoft-Edge-gets-picture-in-picture-mode-on-Android-Oreo-and-later.422326.0.html