Debian 11: How to disable IPv6

[Roger Price <debian@rogerprice.org>]

In a Debian 11 system, I would like to disable IPv6 adapters in order to 
persuade fetchmail to talk to exim4.  The advice generally given is to add a 
line to /etc/sysctl.conf

 net.ipv6.conf.all.disable_ipv6 = 1

and run sysctl -p as root.  With Debian 11 this generates the error message

 sysctl: cannot stat /proc/sys/net/ipv6/conf/all/disable_ipv6: No such file or directory

because directory /proc/sys/net/ipv6 doesn't exist. What is the new way of 
disabling IPv6?

Roger