When I used buster installer (text mode), I was always required to set up network, and always forced to download something during installation, even if I chose not to use a network mirror. My internet connection has been extremely poor, and said downloading (maybe only a few security patches) would cost hours or days to complete. (I really don't want to try to disable all NICs in BIOS.)
I'm looking forward to weekly builds for bullseye, so I could ask my friends who have usable internet connections to download the latest DVD images and post physical media to me, so I could use them as local mirror.