According to this article on itsfoss.com nftables should replace iptables after updating from 20.04 to 20.10. In my case not only is iptables still installed, but also nftables is missing.
Does it imply that the update process has failed and there might be other components missing? Is it much of an issue from security point of view?
The "iptables" package in Ubuntu 20.10 is actually "iptables-nft", which means it controls nf_tables kernel module.
If you want, you can use nftables without problem. Just install it.
I think the reason why it still uses iptables-nft as default management frontend, is that many other ubuntu packages are still heavily depending on iptables.
The images of Ubuntu 20.10 for desktop and server still rely on iftables, nftables is not installed in either of them. Why Canonical has decided to go with iftables once more I don't know. So no, your update went fine in that respect.