My actual problem was that in distance learning sessions I wanted to draw/write something quickly for my students on a sheet of paper, and with auto-mirror it's useless.
Here comes the workaround.
I have an additional (external) USB camera, which is a cheap Logitech C270, and after having installed the Logitech software I discovered that the image is still mirrored in the Teams video session, but NOT in the default Windows camera application. So, I do the following:
I open the Camera App and select the USB camera as its input. Next, I let the Teams share my desktop instead of showing the video connection (which is still handled - in mirror mode - by the built-in front webcam). Now, the students (and myself) can see the non-flipped picture, taken from the Camera App on my desktop. In fact, I can switch back and forth between the desktop mode and the video mode - whenever I want the students to admire my face instead of my drawings.
One can do the same trick with any other external camera and any app that does not mirror the camera input, in contrast to Teams. I know this does not answer your basic question, but may be of some help.