If you are really looking for TTL advice for fw4, you might edit the message title, so that people knowing about that might notice...
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Then back to topic "what is iptables-nft?":
Iptables-nft is a translation layer that aims to translate outdated iptables commands to nft equivalents and then applies them. Depending on the nft components installed and the complexity of the original iptables command, it may succeed or it may fail.
Iptables-nft is not a core part of the fw4 firewall.