None of the machines I've upgraded to focal from bionic have a symlink in /sbin/iptables.
$ namei -l /sbin/iptables
f: /sbin/iptables
drwxr-xr-x root root /
drwxr-xr-x root root sbin iptables - No such file or directory
However you're right that a fresh install does have them. I'm not sure what is supposed to install the symlinks, but it doesn't seem to happen on upgrades.
None of the machines I've upgraded to focal from bionic have a symlink in /sbin/iptables.
$ namei -l /sbin/iptables
iptables - No such file or directory
f: /sbin/iptables
drwxr-xr-x root root /
drwxr-xr-x root root sbin
However you're right that a fresh install does have them. I'm not sure what is supposed to install the symlinks, but it doesn't seem to happen on upgrades.