Yes that was always possible, I myself flashed them all witch Tasmota since I have some other sensor devices with Tasmota, I like to have the same firmware as much as possible, so I can’t check it for you.
If you don’t see this option in the app, you can also access the device by IP from a web browser what gives you some extra option like enabling MQTT (don’t know if it’s still the same, but when I used the Shelly firmware I always had to enable MQTT by the browser, this option isn’t available in the app)