Using classic Outlook for Windows in business environments
this is still the best solution and proof that it works is you can literally click the slider in the top right corner of the mail app to "try new outlook" and laugh as it fails to install it.
Do note that windows updates seem to manage to undo this as I had one recently undo it after weeks of it being gone, but it's a simple short process to redo this, this time i did the additional steps that the OP pointed out about changing allow to deny, and hopefully that prevents future window update undoings, but even if not, it's a quick process to simply redo these steps and restore mail.