ipv6only=on seems to behave the opposite of what it should. I have a default_server listening on all interfaces. It works fine using
listen [::]:80 default_server;
listen 80 default_server;

but if I do
listen [::]:80 default_server ipv6only=on;
listen 80 default_server;

then I get a bunch of errors about already bound addresses on startup. net.ipv6.bindv6only is set to 1.