I just upgraded to Catalina, and Safari does not dynamically adjust my view of a website when I adjust the size of the window. I assume that this is a simple setting that I need to turn on or off, but I can’t figure out how to do it.
Are you sure that website is using responsive design? Have you visited it before on Mojave and it adjusted to the viewport?
I don’t know if this is a Safari bug or not but maybe that website is having some issues.