https://wiki.nftables.org/wiki-nftables/index.php/Netfilter_...
If you know how iptables maps to that diagram you are very likely to be able to quickly understand how nftables does too.
Soooo many systems are still using iptables even though we "should" be using nft everywhere.
If you're going to be a Linux Sys/Net Admin today, you need an understanding of both systems.
That makes no sense. Just because I do not know X, it does not necessarily follow that I am not required to know it, not at all. I might need it for my job, or my future job. I might need it for a Linux distribution I just installed, and so forth. Or perhaps I am already using iptables, but I do not know it.
To your C++03 analogy, I wouldn’t recommend learning C++03, but I also wouldn’t recommend solely learning C++23 either. C++20 and 23 have some really cool stuff in them that can definitely make your code cleaner, but there’s a lot of codebases that are stuck on older versions (at $JOB one of our target platforms is stuck on C++17 and will never get an upgrade so we can’t move the codebase forward until we abandon that kit).
I for instance have never really used iptables in anger, but have lots of experience with nftables and pf. I’ve used both in a professional setting. People can be made aware of iptables, but unless there’s a need to know it, I wouldn’t recommend picking it up now. And you’ll know if you need to learn c++17 or iptables, or python 2.7.