Damn, the S20's viewport is 15px shorter than the minimum width I had in mind. Responsive design is not fun. Will look into making the site more flexible - thanks for your feedback!

Really beautiful UI but unfortunately I can't use it on my phone either. iPhone SE (1st gen.) 320px wide viewport.

I know it's a small phone, but I don't think I've ever come across a web page that blocked me from interacting with it due to the screen size. Regardless of whether you decide it's worth it to support smaller screen sizes I would absolutely recommend removing the page-mask/sorry-prompt. You never know what kind of device/browser your web page will be rendered on and overflowing the viewport is the expected behavior when the viewport is very small.