I haven't looked at that before. But for what it's worth, I confirm that if I navigate to the web app on my phone, navigate to the HTML version, and repeatedly

I haven't looked at that before. But for what it's worth, I confirm that if I navigate to the web app on my phone, navigate to the HTML version, and repeatedly click to ignore gmail's suggestions to use the mobile app, then it is in fact possible to send plaintext email from my phone. I don't think this is a suitable replacement for an email app. For example, this is an extremely inconvenient way to check email, and having to use a different process to check and read email is too much friction. But this does seem like a workaround to send a plaintext email.